You ever have one of those friends, the prankster?
My buddy at work thought he was the reigning prankster, until I shut him down with a mustard-filled Reese's cup.

In this tutorial I'll recreate the process I used in its wretched creation.

You will need:
A Reese's Peanut Butter Cup [preferably individually wrapped]
Small knife for incising.
A food product that completely contrasts peanut butter and chocolate. [I used Feta cheese]
Super glue

Optional:
Ice Pack
Surgical Gloves or similar glove.

[Having to state the obvious: don't give this 'modified' candy to anyone who may have food allergies, intolerances to certain foods, etc]

Step 1: Prep the Patient

1: Prep the Reese's Peanut Butter Cup by carefully removing the packaging at the seems.

Notes:
When I first did this prank the Reese's cup was from the prankee's own bag of individually wrapped chocolates. I unfortunately couldn't obtain an individually wrapped one without purchasing an entire bag. I enjoy them too much to trust myself with a 1/4 pound worth. But if you have an individually wrapped chocolate, the packaging is much easier to remove without damaging it than the twin pack.

Step 2: Prep the Patient

2: Carefully remove the chocolate from the fluted cup. Place chocolate upside-down; base up.

Notes:
I used a knife to peel back the liner.
Once out I placed the chocolate on an ice-pack while I worked on it; to keep it from collapsing, melting, leaving prints, etc. This is where the surgical gloves will come in handy.

Step 3: The Procedure

3: Make a circular incision centered on and half the radius of the base, just breaking through the chocolate.

Notes:
Don't go too deep. I etched through the chocolate first 'cause it was somewhat hard from being on the icepack. Once through the chocolate you can remove the center cut.

Step 4: Extraction

4: Remove the incised section of chocolate and extract peanut butter.

Notes:
I was really tempted to eat the SOB by this point.

Step 5: Transplant

5: Fill with vomitous, edible substance.

Notes:
My original prank Reese's contained mustard, from a packet. I was at work, at my desk, so I went with what I had. This time 'round I was home, with a wider selection. Found a nice garlic, herb feta. Yum, yum. The consistency was perfect.
Fill the hollowed cup with about as much feta as peanut butter that was removed.

Step 6: Close and Patch

6: Replace the plug and begin cosmetic repair.

Notes:
Because I poked through the top I used the second cup for the alogenic graft of chocolate to patch the hole. It's best to let the cup get to room temp. When the chocolate was soft enough I simply ran my index finger around the base incision, smearing the chocolate until it was smooth enough to be overlooked. I did the same for the top repair.

Step 7: Re-wrap

I don't really want to give this nasty thing to anyone, so I dissected the chocolate for a better view.
If you plan on actually giving it to someone, put the Reese's back in the fluted cup then into the original packaging. A few strategic dabs of super glue in the seems, let it dry and wait for the fun.

And actually, the dog had been watching me the whole time so I gave the two halves to him. I'm pretty certain he didn't taste a thing.

Hope you get as good a laugh as I had if you try this on a 'friend'.