For the frosting: In the bowl of a stand mixer fitted with the paddle attachment, cream the butter till smooth.
Add in the powdered sugar, vanilla, and milk. Mix on low till combined.
Turn the mixer up to medium and continue to whip the frosting for a minute or so.
Divide into bowls and color with food colorings. You'll need yellow for the "cheese", red for the "sauce" and tan for the "crust".
For the pepperoni: Combine all ingredients in the bowl of a stand mixer and mix on medium high for 7 to 10 minutes. Add red food coloring.
Place the icing into a piping bag fitted with a medium round tip.
On a large sheet of parchment paper, pipe a bunch of 1 inch red circle shapes. Let set till hardened and gently peel off from the parchment paper.
To assemble: Using a long sharp knife, slice a 13 x 9 inch cake of your choice into a large triangle.
With a pale tan colored buttercream, frost the entire cake. First do a very thin crumb coat. Chill in the fridge for about 30 minutes. Then do another coat of buttercream.
Spread red colored buttercream over the top and slightly over the sides of the cake for the tomato sauce, leaving the “crust” the dough color. Chill.
Finish off with your yellow buttercream for the cheese, leaving some of the red “sauce” showing around the edges.
You can also add a little darker brown buttercream to the “crust” to add some more dimension.
Top with some of the royal icing pepperoni. You can slice some of the pepperoni in half using a sharp knife to place along the edges of the pizza slice.