Preheat oven to 350°F (175°C). Grease and flour two 9-inch cake pans or line with parchment paper.In a large bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.In another bowl, whisk eggs, milk, vegetable oil, and vanilla extract. Slowly mix into dry ingredients until smooth.Gradually add boiling water, stirring until batter is thin and smooth.Pour batter evenly into prepared pans. Bake 30–35 minutes, or until a toothpick comes out clean. Cool in pans 10 minutes, then transfer to a wire rack.For caramel: Melt 1 cup sugar in a saucepan until golden. Stir in butter, then slowly whisk in heavy cream. Cool slightly.For ganache: Heat 1 cup heavy cream until simmering. Pour over chopped chocolate, let sit 2 minutes, then stir until smooth. Stir in 2 tbsp butter.Assemble cake: Place first layer on plate, spread caramel on top. Add second layer. Pour ganache over cake, allowing drips down sides. Optionally drizzle extra caramel.Chill 30 minutes before slicing for cleaner cuts. Decorate with crushed Milky Way bars if desired.