1. Preheat the oven to 350°F and grease three 8-inch round cake pans. Line the bottoms with parchment paper.

2. In a medium bowl, whisk together the flour, baking powder, and salt.

3. In a large bowl, beat 1 cup butter with the granulated sugar and brown sugar until light and fluffy.

4. Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.

5. Add the dry ingredients in three additions, alternating with the milk and sour cream, mixing just until combined.

6. Divide the batter evenly among the prepared pans and bake for 22 to 26 minutes, or until a toothpick inserted in the center comes out clean.

7. Cool the cakes in the pans for 10 minutes, then transfer to wire racks to cool completely.

8. To make the frosting, beat the remaining 1 cup butter until creamy. Add the powdered sugar, 1/2 cup caramel sauce, heavy cream, and 1/4 teaspoon salt. Beat until fluffy and spreadable.

9. Place one cake layer on a cake stand, spread frosting on top, then repeat with the remaining layers. Apply a thin crumb coat and chill for 15 minutes.

10. Frost the outside of the cake with the remaining caramel buttercream.

11. Warm the remaining 1/4 cup caramel sauce slightly and spoon it around the edges to create a drip effect.

12. Finish with swirls of frosting and sprinkle the top with toffee bits or chopped caramel candy before slicing.