Prepare the Cake Layers:

1. Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
2. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
3. In a large mixing bowl, beat the butter and sugar together until light and fluffy.
4. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
5. Gradually add the dry ingredients to the butter mixture, alternating with the buttermilk, starting and ending with the dry ingredients. Mix until just combined.
6. Divide the batter evenly between the prepared cake pans.
7. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
8. Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

Make the Caramel Pecan Filling:

1. In a medium saucepan, combine the brown sugar, butter, and heavy cream over medium heat.
2. Stir continuously until the mixture thickens and comes to a gentle boil, about 5-7 minutes.
3. Remove from heat and stir in the vanilla extract and chopped pecans. Let the filling cool until it’s thick enough to spread.

Assemble the Cake:

1. Once the cake layers have cooled, place one layer on a serving plate.
2. Spread a generous amount of the caramel pecan filling over the top of the first layer.
3. Place the second cake layer on top and repeat with the remaining caramel filling between the layers.

Prepare the Caramel Drizzle:

1. In a saucepan, melt the sugar over medium heat until it turns a deep amber color, swirling the pan occasionally.
2. Add the butter and stir until fully combined.
3. Slowly pour in the heavy cream while stirring constantly. Continue to cook for 1-2 minutes until smooth and thick.
4. Remove from heat and stir in the vanilla extract and a pinch of salt. Let the caramel cool slightly before drizzling over the cake.

Decorate and Serve:

1. Drizzle the cooled caramel sauce over the top of the cake, letting it drip down the sides.
2. Garnish with whole pecans on top for a beautiful presentation.
3. Chill the cake for at least 30 minutes before serving to allow the layers to set.