Prepare two pieces of parchment paper to fit your baking sheet.
In the bowl of a stand mixer combine the sugar, butter, salt and baking powder. Blend until fluffy, about 2 minutes
Add the egg and vanilla and mix on medium until blended.
Add the flour and cocoa and mix on low until the dry ingredients are incorporated.
Add the food color, if using, and beat on medium until all ingredients are blended.
Wrap the dough in plastic wrap and refrigerate for 30 minutes.
Preheat oven to 350 degrees.
Lightly dust a piece of parchment paper with a mixture of cocoa powder and flour. Roll out half the dough to 1/8 inch thickness. Cut into heart shaped cookies using a 2 1/2 inch cookie cutter.
Bake for 8-10 minutes taking care not to burn. If you can smell chocolate, they are done. Allow the cookies to cool on a wire rack then refrigerate until ready to assemble the stack cakes.
(May be made 1 day ahead)