Preheat the oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.

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

In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.

Add eggs one at a time, beating well after each addition. Mix in vanilla extract.

Alternate adding the flour mixture and buttermilk to the butter mixture, beginning and ending with flour. Mix until just combined.

Divide the cake batter evenly among the prepared pans and smooth the tops.

Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.

While the cakes bake, prepare the pecan filling by whisking together corn syrup, brown sugar, eggs, melted butter, and vanilla in a bowl.

Fold in chopped pecans into the filling mixture.

Once the cakes are cooled, carefully spread the pecan filling evenly over the top of two cake layers.

Stack the cake layers with pecan filling in between, then frost the entire cake with buttercream frosting.

To make the frosting, beat butter until creamy, gradually add powdered sugar, then mix in heavy cream and vanilla until smooth and fluffy.

Garnish the top of the cake with toasted chopped pecans if desired.

Chill the cake for at least 30 minutes before serving to set the frosting.