Since I have access to a full shop of tools that most people don't have, instead of just explaining how I made the Axe, I'm going to go through the methodology behind how I planned the manufacturing, the resources I used, and my thoughts at different steps. My goal for this instructable is to give you all of the information to make your own whacky chocolate creations from a materials science perspective.

Supplies

Something to melt chocolate

Sous Vide cooker (what I used) OR
a double boiler set up

Different Types of Chocolate

Pick a range of chocolates depending on what outcome you want
45-100% chocolate
Don't use bakers chocolate - most baker's chocolate has removed or substituted its cocoa butter

Mold Stuff (If you don't want to buy something)

MDF board
Shellac
2 part food safe epoxy
1/2 inch dowel rod

Step 1: I Wanna Build an Axe of Chocolate!! (Define What Your Idea Is and Quantify It in Concrete Terms.)

I wanted my chocolate axe to:

Chop things: Hard enough to pierce a watermelons surface, and tough enough not to shatter

Be edible: I want to be able to eat and have the chocolate be enjoyable

So hard, tough, enjoyable. I used Google Scholar to find papers on fracture toughness and tensile strength of chocolate for the first two objectives, and I started testing different cocoa content chocolates for suitable flavor - more on that later. I used those attributes to design the axe. Luckily, it all relates back to one thing: cocoa butter.

Step 2: Cuckoo for Cocoa Butter

Cocoa butter is the substance that makes a great chocolate bar great. It makes a Hershey's bar shiny and gives it that nice snap when you break it in half. It's also why leaving a chocolate bar in a car always makes it dull and bendy. This happens because chocolate, or more specifically, cocoa butter, is a polymorphific material. That means it has multiple crystal formations occuring inside its lattice at the same time. They are six types of crystal formations, numbered I-VI. Above is an image from CompoundChem to help explaining the different crystal formations and their physical properties along with their melting points.

This leads me to why I said not to use baker's chocolate. In research papers and information on different candy bars, cocoa butter and cocoa mass (both solids and butter) are commonly used descriptors but are not interchangeable. Chocolate used in candies and snacks is around 20-40% cocoa butter, with the remainder 60-80% of the cocoa mass being cocoa solids. Thus, a Hershey's Extra Special Dark chocolate bar, which is 60% unsweetened chocolate, could mean that the whole bar contains 12-24% cocoa butter. Baker's chocolate may be 90% or even 95% cocoa solids, with little to no cocoa butter. Conversely, 90-100% cocoa chocolate bars may contain up to 40% total volume cocoa butter.

All of that to say, the higher the cocoa butter content, the better the mechanical properties. (references below)

Step 3: THIS AINT THE TERMS AND CONDITIONS, ACTUALLY READ THIS

This is the most important section of this entire instructable. For the love of all that is holy, if you intend to eat whatever you make, do not use 100% cocoa candy bars.

I repeat.

Don't do it. Listen to your buddy Strange, don't do it.

I fed 90% to my family, and I was uninvited to the next 3 Christmases.

My neice had been stealing my food for months, but after leaving that out, she left me a handwritten apology and never touched my food again.

One bite of high percentage cocoa chocolate tastes like someone concentrated every negative emotion you've ever felt, with the added after taste of a fear of mannequins. I know, it doesn't make sense until you try it.

I chose Dove's 60% dark chocolate because they didn't taste like fear and regret, while also having nice little motivational phrases to help forget about the earlier chocolate taste tests.

Step 4: Design or Prepare Your Mold.

I wanted something double bladed because it is more stereotypically barbarian. I made the design in Fusion 360. I created a positive on my CNC router with MDF, gave it multiple coatings of shellac, and then filed it with two-part silicone.

I used shellac because its nontoxic when dry and has about 3% wax, so I didn't actually use a mold release.

Little tidbit to ruin your day: candy companies actually use shellac to coat certain items like Jelly Belly jelly beans. So next time you're eating jelly beans, remember, you just ate bug secretions.

Step 5: Heat Treating Chocolate and Pouring

The idea is to heat the chocolate until all the cocoa butter is melted, creating a clean slate. Then, the chocolate is cooled down to 80F so that types IV and V crystals form. The chocolate is next heated to 90F so that the type IV crystals melt and the type V crystals act as seed crystals, increasing the speed at which more type V crystals form.

So, here is what I did.

Unwrap 3 pounds of Dove 60% Chocolates and put a handful (about 5) aside. The majority went into a plastic bag and into the Sous Vide cooker.
Bring chocolate to 120F.
Drop it down to 80F by putting ice in the Sous Vide cooker.
Heat cooker back up to 90F (for 3 hours because I got distracted)

You can test the chocolate by dipping a knife in it and letting it cool at room temperature, about 5 minutes, if it is shiny and you can touch it with your fingers coming away clean it's ready. I assume this works but can't verify because Louisiana is hot enough that alligators sprint across black top during the day instead of crawling. It's so hot that my skin temperature is above the melting point of type V chocolate. My test of whether my chocolate was tempered or not was if it snapped and SOUNDED LIKE A STEVEN SEAGAL FINISHING MOVE.

Ahem, next, tape up the mold and get ready to pour.

About 10 minutes before pouring into the mold, I finely chopped the handful of chocolates I had set aside, mixed them into the molten chocolate, and was front seat on the struggle bus because I didn't use a funnel. Use a funnel, guys. Adding the chopped chocolate added more type V crystals because I believe in redundancy because store-bought chocolate is typically already tempered.

After getting the mold filled, grab the dowel rod, plant it like a flag to your chocolate dreams, and leave it alone overnight at room temperature.

Step 6: Time to Make It Stronger and CHOP STUFF

After leaving the axe out to cool to room temperature overnight, I froze it in the freezer for 8 hours to increase its strength as much as possible. Here is another aspect that I didn't consider: Louisiana is hot right now, averaging 90F outside. That meant my frantic setting up of the skit looked like a Willy Wonka viking fever dream to my family, but at the end of the day, the axe murdered the fruit and was delicious.

Step 7: Results and References

I'm including several references and documents that you may enjoy reading if you plan to make something yourself. I hope this Instructable helps inspire you to make something crazy and fantastic with chocolate. I was leaning heavily into the entertainment aspect of this, but there are additional teaching documents included if you want to be "traditional" with your science communication.