Preheat oven to 375°F (350F if you don't want domed cupcakes).
Line a cupcake tray with liners and set aside.
Stir sugar with flour, cocoa, baking powder, baking soda and salt in a large bowl.
Add eggs, milk, oil and vanilla, and beat well.
Slowly stir in boiling water until you reach a runny consistency.
Portion out into cupcake liners - filling 3/4 of the way through.
Bake for 18-22 minutes and allow to cool in the pan for one minute before removing to a cooling rack.
For the frosting
Beat the butter and sugar on medium-high speed until light and incorporated. Slowly add the vanilla and heavy cream, and adjust the consistency by adding more icing sugar (for stiffer) or heavy cream (for looser).
Spread the frosting onto the cupcakes using an offset spatula, then drizzle the chocolate sundae sauce overtop. Top with remaining sundae toppings and serve!