I created a basic tutorial for how to draw with melted chocolate (here). This tutorial represents a version tailored to this specific cake.

Materials

9x13 inch frosted cake
Colored Candy melts
Tooth pick
Printer

Step 1: Bake Your Cake

The cake in the final picture features 2 9x13 frosted layers. The chocolate piece is hard so it is important that the top of the cake is level. To achieve this make sure that you flip your top cake layer upside-down so that it is flat (alternatively you could level the cake with a knife by cutting off the dome)

Step 2: Create a Jackson Pollock Design

There are three ways that you can do this:

Trace a Jackson Pollock: This is the most advanced and time consuming option. I printed a Jackson Pollock image and flipped it horizontally and then traced the image with chocolate so create a replica (Detailed instructions are here). I then covered the who piece with white chocolate. Once it was dry I removed the chocolate piece and went over many of the paint splatters to create texture.
Freeform Chocolate Piece: This technique is much easier. You draw a rectangle the size you want your chocolate piece to be and take it to a clipboard. Temper the candy melts until smooth and runny. Drip your toothpick into the chocolate and splatter onto your paper. Blowing with a small handheld fan can help to disperse the chocolate. After all the splatters dry cover with white chocolate.
Dripped Chocolate: The easiest way it to temper the chocolate and splatter it directly onto the cake. Do not over cook or this will not work. I like the texture that is easily achieved with this technique.