Pecan Pie Layer Cake

1 1/4 cup salted butter, room temperature
1 1/4 cups shortening
10 cups powdered sugar
1 tbsp vanilla extract
1 tsp ground cinnamon
5–6 tbsp water or milk
Additional:

Pecan halves, for decorating
Instructions:

To Make the Pecan Pie Filling:

In a saucepan, combine all filling ingredients except pecans. Heat over medium heat, stirring constantly until it begins to boil.
Add chopped pecans and allow to boil for 30 seconds.
Remove from heat and refrigerate until cool and thickened.
To Make the Cake Layers:
4. Preheat oven to 350°F (176°C). Grease and line three 8-inch cake pans with parchment paper.

In a large mixing bowl, cream butter and sugars until light and fluffy.
Add sour cream and vanilla extract, mixing until well combined.
Gradually add egg whites, mixing well after each addition.
In a separate bowl, combine dry ingredients. Alternate adding dry ingredients and milk mixture to the batter, mixing until well combined.
Divide batter evenly among prepared pans and bake for 21-23 minutes, or until a toothpick inserted into the center comes out clean.
Cool cakes in pans for 2-3 minutes, then transfer to wire racks to cool completely.
To Make the Frosting and Assemble Cake:
11. In a large mixer bowl, combine butter and shortening until smooth.