Heat oven to 350 degrees. Arrange two racks in center of oven. Butter three 8-by-2-inch round cake pans; line bottoms with parchment. Dust bottoms and sides of pans with cocoa powder; tap out any excess. Sift cocoa and espresso powder into a medium bowl, and whisk in boiling water. Set aside to cool.
In the bowl of an electric mixer fitted with the paddle attachment, cream butter on low speed until light and fluffy. Gradually beat in sugar until light and fluffy, 3 to 4 minutes, scraping down sides twice. Beat in vanilla. Drizzle in eggs, a little at a time, beating between each addition until the batter is no longer slick, scraping down the sides twice.
In a large bowl, sift together flour, baking soda, and salt. Whisk milk and Bailey's into reserved cocoa mixture. With mixer on low speed, alternately add flour and cocoa mixtures to the batter, a little of each at a time, starting and ending with flour mixture.
Divide batter evenly among the three prepared pans. Bake until a cake tester inserted into center of each layer comes out clean, 35 to 45 minutes, rotating the pans for even baking. Transfer layers to wire racks; let cool, 15 minutes. Turn out cakes, and return to racks, tops up, until completely cool.
Remove parchment from bottoms of cakes. Sliceone cake layer in half, from top to bottom, set aside. Place one full cake layer on a serving platter; spread 1 cup chocolate frosting over the top. Add the second full cake layer, and spread with another 1 cup frosting. Add one of the half layers, spread with frosting and top with remaining half cake layer. Using a serrated knife, carve the top two layers on the diagonal to create a hill shape. Cover outside of cake with the remaining 3 cups frosting. Decorate with chocolate graham cracker crumbs, malted milk balls, and trucks! Serve with Rocky Road Ice Cream!