Making the Cake Layers
Preheat oven to 350F (175C). Grease and flour seven 8-inch cake pans. Alternatively, use fewer pans and bake in batches.
In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
In a large bowl, cream together butter and sugar until light and fluffy.
Beat in eggs one at a time, then stir in vanilla extract.
Gradually add dry ingredients to the wet ingredients, alternating with buttermilk, beginning and ending with dry ingredients. Mix until just combined. Do not overmix.
Divide batter evenly among prepared cake pans.
Bake for 12-15 minutes, or until a wooden skewer inserted into the center comes out clean.
Let cool in pans for 10 minutes, then invert onto a wire rack to cool completely.
Making the Caramel Topping
In a heavy-bottomed saucepan, combine sugar and butter. Cook over medium heat, stirring constantly, until sugar is melted and mixture is smooth.
Bring to a boil and cook, without stirring, until the mixture turns a deep amber color (about 5-7 minutes). Watch carefully to prevent burning.
Remove from heat and slowly stir in heavy cream. Be careful, as the mixture will bubble vigorously.
Stir in salt and vanilla extract.
Let the caramel cool slightly, until it thickens enough to spread.
Assembling the Cake
Place one cake layer on a serving plate.
Spread a thin layer of caramel over the cake layer.
Repeat with remaining cake layers and caramel, stacking them one on top of another.
Pour the remaining caramel over the top layer, allowing it to drip down the sides.
Let the cake sit for at least an hour to allow the caramel to set before serving.
Tips and Substitutions:
For a richer flavor, use salted butter in the caramel topping. A pinch of sea salt sprinkled over the finished cake enhances the sweet and savory notes.
If you don't have buttermilk, you can make a substitute by adding 1 tablespoon of lemon juice or white vinegar to a measuring cup and filling the rest with milk. Let it sit for 5 minutes before using.
Pecans or walnuts can be added to the caramel for a nutty variation.