Preheat oven to 350 degrees.
Fill a muffin tin with 12 cupcake liners and set aside.
Combine cocoa powder and water in a bowl and mix until smooth. Fold in mayonnaise. (Make sure cocoa mixture is not too hot to prevent breaking of mayonnaise). Add vanilla and whisk until smooth.
In mixer, combine sugar, cake flour, baking soda and salt and mix slowly until well incorporated. Add wet cocoa mixture and mix on medium-high until well blended. Scrape down the sides and blend for 30 seconds.
Fill the cupcake liners ¾ way full of batter. Bake for 30 minutes or until toothpick comes out clean. Remove from oven and allow to cool on a rack.
Slice each cupcake in half, horizontally.
Set out the cupcake halves, the chocolate mousse or frosting, the whipped topping, and assorted candy pieces.
Kids build the sandwiches by spreading the bottom half of a cupcake with mousse/frosting or whipped topping, sprinkling on candy pieces and topping with the other half of the cupcake.