Heat oven to 350°F (175°C). Grease and lightly dust two 9-inch cake pans with cocoa powder.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt in a large bowl.
Beat eggs, milk, oil, and vanilla until smooth.
Add wet to dry ingredients and mix until combined. Slowly stir in boiling water; batter will thin.
Divide batter evenly and bake 28–32 minutes. Toothpick should come out with a few crumbs.
Cool in pans 10 minutes, then transfer to wire racks until completely cool.
Beat butter until fluffy. Gradually add powdered sugar, then caramel and vanilla. Add heavy cream to reach desired consistency.
Layer cake with frosting and half the toffee bits between layers. Frost entire cake.
Drizzle caramel and ganache over cake. Sprinkle remaining toffee bits on top and sides.