1. Prepare the Crust:

1. Preheat the oven to 350°F (175°C). Grease and flour three 9-inch round cake pans.
2. In a large bowl, cream together the butter and sugar until light and fluffy. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla.
3. In another bowl, combine the flour, baking soda, and salt. Add the dry ingredients to the creamed mixture alternately with the buttermilk. Fold in the pecans.
4. Divide the batter equally among the prepared pans and bake for 25 to 30 minutes, or until a toothpick inserted into the center comes out clean.
5. Meanwhile, prepare the pecan pie filling by combining the corn syrup, brown sugar, eggs, and vanilla in a saucepan. Cook over medium heat, stirring constantly, until the mixture thickens. Let cool slightly.
6. Once the cakes are done and cooled, spread the pecan pie filling between the layers and over the top of the cake.