Bake two 8-inch round cakes at 350°F for 25–30 minutes. Cool completely on wire racks.
Level cakes and stack with 1 cup buttercream between layers. Chill 20 minutes.
Crumb-coat with 1 cup buttercream, chill 15 minutes. Smooth remaining buttercream over cake.
Warm heavy cream until simmering, pour over chopped chocolate and stir until glossy. Cool 5 minutes.
Pour chocolate ganache around the cake edge so it drips down; pour a little on center.
Immediately press candies and sprinkles onto the top and sides where you want them to stick.
Chill 30 minutes to set ganache and candies before serving.