Preheat oven to 350 degrees F.
Fill a muffin tin with 12 cupcake liners and set aside.
Combine cocoa powder and water and mix until smooth. Fold in mayonnaise. (Make sure cocoa mixture is not to 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, mix on medium-high until well blended. Scrape down the sides and blend for 30 seconds.
Fill the cupcake liners 3/4 way full of batter. Bake for 30 minutes or until toothpick comes out clean. Remove from oven and allow to cool on rack.
Slice each cupcake in half, horizontally, and serve the halves with chocolate mousse, whipped cream, and assorted candy toppings. Kids build the sandwiches by spreading one half of a cupcake with mousse or whipped cream, sprinkling on candy toppings and placing other half of cupcake on top.
In a microwave safe bowl, combine the chopped chocolate and the butter. Heat in the microwave until melted, about 35 seconds depending on your microwave, and stir to melt completely. Whisk in the egg yolks and set aside to cool.
Pour the cream into a chilled bowl of a mixer and beat on medium speed until it starts to thicken. Add the sugar and vanilla and whisk until the cream starts to form stiff peaks. Carefully fold the whipped cream into the cooled chocolate mixture. Serve immediately or cover and chill until ready to serve.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.