For the cake: Position a rack in the center of the oven and preheat the oven to 350 degrees F. Coat two 8-inch-round cake pans with cooking spray and line the bottoms with parchment paper rounds.
Sift the flour, cocoa powder, baking powder, baking soda and salt into a medium bowl and set aside.
In a small bowl, combine the hot coffee and semisweet chocolate. Allow the coffee to melt the chocolate, then cool the mixture to warm, not hot. Stir to blend the chocolate into the coffee. Add the buttermilk and vanilla and stir again. Set aside.
Whisk together the oil, eggs and egg yolk in the bowl of a stand mixer on medium high, about 1 minute. Reduce the speed to low and add the coffee mixture. Gradually add the flour mixture and mix on low until well blended, about 1 minute. Give the batter a 20-second burst on medium-high speed.
Divide the batter evenly among the prepared pans and bake on the center rack until a toothpick inserted into the centers of the cakes comes out clean, 35 to 40 minutes. Allow to cool for about 30 minutes in the pans, then remove from the pans and cool completely on a wire rack. Carefully double-wrap the cooled cakes in plastic wrap and refrigerate 2 to 3 hours or until you are ready to decorate.
For the buttercream: Place the malted milk balls in a resealable bag and seal. Break the candies into small chunks with the back of a spoon and set aside.
Beat the butter in the bowl of a stand mixer on medium speed until nice and smooth, about 1 minute. On low speed, gradually add the confectioners' sugar until fully incorporated. Add the malted milk powder, vanilla and salt. Beat on medium speed until well combined, 2 minutes. (For a firmer buttercream, add a little more confectioners' sugar. For a softer buttercream, add a little water a tablespoon at a time until it reaches the desired consistency.) In a small bowl, mix a quarter of the buttercream with the crushed malted milk balls. Store in airtight containers until ready to use.
To assemble the cake: Remove the plastic wrap from the cakes. Using a long serrated knife, carefully cut off the domed sections of each layer of cake. (Discard the scraps or give to eager family and friends.) Place one layer of the cake on a plate or disposable cardboard cake circle. Using a medium-size offset spatula, spread about a 1/2-inch layer of the buttercream with candy pieces onto the top of the cake layer, leaving about half an inch of space between the buttercream and the edge of the cake. Carefully place the other layer on top of the one with the candy buttercream. Using the plain buttercream (without candy pieces), spread a thin layer of buttercream all the way around the entire cake. Chill the cake for about 30 minutes. This will act as a crumb coat and lock in all of the loose crumbs before adding the final layer of icing. Remove the cake from the refrigerator and, using your spatula, spread another layer of buttercream on the sides and top of the cake. Crush additional malted milk candies and sprinkle on top of the cake.
This recipe was created by a contestant during a cooking competition. It has not been tested for home use.