Preheat your oven to 350°F (175°C) and grease and flour two 9-inch round cake pans.

In a large mixing bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt. Whisk until well mixed.

In another bowl, whisk together eggs, milk, vegetable oil, and vanilla. Pour this into the dry ingredients and mix until smooth. Carefully add boiling water to create a thin batter.

Distribute the batter evenly in the prepared pans and bake for 30-35 minutes, or until a toothpick comes out clean. Cool in pans for 10 minutes before transferring to wire racks.

In a saucepan over medium heat, melt sugar until it caramelizes (5-7 minutes). Add butter, heavy cream, vanilla, and salt, stir until combined, and let cool.

Beat the butter until creamy, gradually adding powdered sugar and cocoa powder. Then mix in vanilla and heavy cream until fluffy.

Once cakes are cool, place one layer on a plate, drizzle with caramel sauce, and add a layer of frosting. Top with the second layer, frost the top and sides, and drizzle with more caramel.