Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt. Mix well.
Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed for 2 minutes until well combined.
Carefully stir in the boiling water (the batter will be thin). Pour the batter evenly into the prepared cake pans.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then remove from pans and cool completely on wire racks.
For the frosting, beat the softened butter in a large bowl until creamy. Gradually add the powdered sugar, mixing well.
Add the salted caramel sauce, vanilla extract, and sea salt. Beat until smooth and fluffy.
Once the cakes are completely cooled, place one layer on a serving plate. Spread a layer of frosting on top, then place the second layer on top. Frost the top and sides of the cake with the remaining frosting.
Drizzle additional salted caramel sauce over the top of the frosted cake, if desired.