Preheat oven to 350 degrees. Make the cake: Coat two 9-inch cake pans with cooking spray. Line bottoms with parchment, and coat with spray. Dust with all-purpose flour, tapping out excess. Whisk together flours, baking soda, and 3/4 teaspoon salt.
Beat butter and sugar with a mixer on medium-high speed until pale and fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Reduce speed to low. Beat in flour mixture in 3 additions, alternating with buttermilk, beginning and ending with flour. Beat in melted chocolate. Divide batter between pans, and smooth tops using an offset spatula.
Bake cakes until a toothpick inserted into the centers comes out clean, 25 to 30 minutes. Let cool in pans set on wire racks for 30 minutes. Turn out cakes onto racks, remove parchment, and let cool completely.
Make the ganache: Bring cream to a gentle simmer in a small saucepan over medium-high
heat. Pour over chopped chocolates and butter in a bowl. Let stand for 5 minutes. Whisk until smooth and shiny. Let stand until thickened, about 40 minutes. Transfer 1 3/4 cups ganache to a bowl, and fold in malt balls; reserve remaining ganache.
Transfer 1 cake to a serving platter (top side up). Spread ganache-malt ball filling on top, and top with remaining cake (top side up). Refrigerate until set, about 15 minutes. Cut a large wedge from cake to serve on another plate. Pour remaining ganache (reheat if necessary) over cake and wedge, and let drip down sides and into cut center of cake. Garnish cake and platter with rubber cockroaches.
Decorated cake can be refrigerated for up to 3 days, but the ganache will not be as shiny. Bring to room temperature before serving.