Once cake is baked, let cool for about 20 minutes and then remove from pan. Let cool completely.
Next, make the buttercream. In the bowl of a stand mixer fitted with the paddle attachment, cream together the shortening and butter till smooth.
Add in the cocoa powder and powdered sugar. Mix. Add in the vanilla and then the milk as needed to get a light fluffy texture. Continue to beat on medium for 1 more minute.
Spread an even layer of frosting over the entire cake and smooth with an offset spatula.
Using a knife or spatula, draw a line across the middle in the frosting.
Top the cake with the rainbow chocolate chip sprinkles.