1. Preheat the oven to 350 degrees. Line (3) 6-inch cake pans with parchment paper and spray with baking spray containing flour on the bottom of the pans. Alternatively, you can use 2 8-inch cake pans.

2. In a small bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

3. In a large mixing bowl, beat the butter, sugar, and brown sugar together until light and fluffy (about 2-3 minutes). Add the eggs and vanilla, and continue beating until creamy.

4. Alternately add the flour mixture and the buttermilk, beginning and ending with the flour, scraping the sides of the bowl until all ingredients are thoroughly combined. 

5. Divide batter equally among the prepared cake pans. You can use a small kitchen scale to ensure equal amounts in each pan.

6. Place the cake pans in the preheated oven and bake for 30-35 minutes or until a wooden pick or knife comes out clean when placed in the center of the cake.

7. Let the cakes cool in the pans for 10 minutes before transferring them to a rack to cool completely.

1. In a large saucepan, combine the pecans, water, brown sugar, cornstarch, and egg yolk. Whisk over low to medium heat until it boils. 

2. Let it cook for 1-2 minutes, then remove from heat and add the butter and vanilla.

3. Let it cool completely before adding it to the cake.

1. Using an electric mixer or stand mixer, beat the butter until it becomes lighter in color and creamy (about 2 minutes) in a large bowl.

2 Add the powdered sugar and vanilla extract and beat until smooth and creamy. If necessary, add 1 to 2 tablespoons of milk to make it easy to spread.

1. If needed, trim the tops of the cakes to create a flat surface. Place one cake layer on a cake stand or serving plate.

2. Pipe a circle of the buttercream frosting around the edge of the cake to create a barrier for the filling. Spoon half of the pecan filling into the center and spread it evenly.

3. Add the second layer and repeat the process.

4. Once the layers are stacked, apply a thin layer of frosting over the entire cake. Chill the cake in the fridge for about 30 minutes to set the frosting, then add another layer for a smooth finish.

5. If desired, pipe extra swirls on top of the cake and place pecans on each peak. Slice and enjoy!

6. Store leftovers in an airtight container for up to 3 days.