Prepare the cake batter:

• Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
• In a medium bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
• In a large bowl, beat together the granulated sugar, brown sugar, eggs, and vanilla extract until smooth and well combined.
• Add the dry ingredients alternately with the milk and oil, mixing just until combined.
• Finally, add the boiling water and stir until the batter is smooth (it will be thin).

Bake the cake:

• Pour the batter evenly into the prepared cake pans.
• Bake for 30-35 minutes or until a toothpick inserted into the center of the cakes comes out clean.
• Remove the cakes from the oven and let them cool in the pans for 10 minutes before transferring to a wire rack to cool completely.

Make the salted caramel sauce (if homemade):

• In a medium saucepan, melt the butter over medium heat. Stir in the brown sugar and bring it to a boil.
• Once boiling, allow it to cook for about 2 minutes, stirring constantly.
• Carefully add the heavy cream (it will bubble), then reduce the heat and continue stirring until the mixture thickens slightly, about 3 minutes.
• Stir in the vanilla extract and a pinch of salt, then remove from heat and let cool to room temperature.

Make the salted caramel buttercream:

• In a large mixing bowl, beat the softened butter with an electric mixer until creamy and smooth.
• Gradually add the powdered sugar, 1 cup at a time, beating well between each addition.
• Add the cooled salted caramel sauce and vanilla extract, and beat until smooth.
• Taste the buttercream and, if desired, add a pinch of salt to balance the sweetness of the caramel.

Assemble the cake:

• Once the cake layers are completely cooled, place one layer on a cake stand or serving plate.
• Spread a generous layer of salted caramel buttercream on top of the first cake layer.
• Place the second cake layer on top and frost the entire cake with the remaining buttercream.
• Optionally, drizzle extra salted caramel sauce on top for added flavor and decoration.

Serve and enjoy:

• Allow the cake to set for about 15-20 minutes before slicing to let the frosting firm up. Serve and enjoy!