Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans and line the bottoms with parchment paper.

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

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3 to 4 minutes. Add the eggs one at a time, mixing well after each. Stir in the vanilla extract.

With the mixer on low, alternate adding the dry ingredients and the buttermilk in three parts, starting and ending with the dry mix.

Divide the batter evenly between the prepared pans and bake for 25 to 30 minutes, or until a toothpick inserted in the center comes out clean.

Let the cakes cool in their pans for 10 minutes before turning them out onto wire racks to cool completely.

In a medium saucepan, melt the butter over medium heat. Stir in the brown sugar and milk, and bring to a gentle boil. Simmer for 2 minutes, then remove from heat and add the vanilla.

Let the caramel cool slightly, about 5 to 7 minutes, then whisk in the powdered sugar until smooth.

Place one cooled cake layer on a serving plate. Spread half of the frosting over the top.

Add the second layer and pour the remaining frosting on top, letting it drape over the sides.