Prepare the Pecan Pie Filling:

1. In a saucepan over medium heat, combine brown sugar, corn syrup, heavy cream, and butter. Stir until the butter melts and the mixture begins to bubble.
2. In a separate bowl, whisk egg yolks. Slowly add a few spoonfuls of the hot mixture to the yolks, whisking constantly.
3. Pour the tempered yolks back into the saucepan and cook for another 2-3 minutes, stirring constantly until thickened.
4. Remove from heat, stir in vanilla extract and pecans. Let cool completely before using.

Make the Cake Layers:

1. Preheat oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.
2. In a bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
3. In a separate large bowl, beat butter, granulated sugar, and brown sugar until light and fluffy. Add eggs one at a time, mixing well.
4. Add vanilla and mix. Alternate adding the flour mixture and buttermilk, beginning and ending with flour.
5. Divide batter evenly between the pans and bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
6. Let cakes cool in pans for 10 minutes, then transfer to a wire rack to cool completely.

Prepare the Butter Pecan Frosting:

1. In a large bowl, beat butter until creamy. Gradually add powdered sugar, heavy cream, vanilla, and salt. Beat until smooth and fluffy.
2. Fold in toasted pecans.

Assemble the Cake:

1. Place one cake layer on a serving plate. Spread half of the pecan pie filling over it.
2. Repeat with the second cake layer and the remaining filling.
3. Top with the third cake layer and frost the entire cake with butter pecan frosting.
4. Garnish with extra pecans if desired. Refrigerate for at least an hour before serving for best results.