Make One-Bowl Chocolate Cake Batter and bake following the layer cake instructions.
Make the frosting: Put the chocolate in a microwave-safe bowl and microwave in 30-second intervals, stirring, until melted. Let cool slightly. Stir together the cocoa powder and vanilla in a small bowl. Beat the butter and salt in a large bowl with a mixer on medium speed until fluffy and smooth, 1 to 2 minutes. Add the melted chocolate and beat until combined. Gradually add the confectioners' sugar and beat until smooth. Add the cocoa mixture. Increase the speed to medium high and beat until smooth and fluffy, 1 to 2 minutes.
Assemble the cake: Put 1 cake layer on a cake stand or serving plate and spread with 1 heaping cup frosting. Sprinkle with the chopped candy and top with the other cake layer. Spread the remaining frosting on the top and sides of the cake. Refrigerate until set, about 30 minutes. Top with more candy.