Preheat oven to 350. Line cupcake pan with cupcake liners and set aside.
In a large bowl beat butter and sugar with a mixer on medium speed until fluffy, 3-4 minutes. Add eggs, one at a time, beating well after each addition.
In a small bowl, stir together flour, baking powder, and salt. With mixer on low speed add flour mixture to butter mixture in thirds, alternating with buttermilk, beginning and ending with flour mixture. Beat in vanilla. Pour batter into prepared cupcake liners. Bake for 12-15 minutes. Test with a toothpick inserted into the center of cupcake if it comes out clean they are done.
Let cupcakes cool when removed from oven.
While the cupcakes are in the oven prepare the apple butter filling.
Apple Butter Filling
In a large saucepan whisk together egg yolks, sugar, and melted butter. Bring to a boil over medium heat stirring constantly until a candy thermometer reaches 160 degrees. Stir in apple butter. Remove from heat and let cool.
After cupcakes have cooled used a melon baller or small spoon to scoop out the center of the cupcakes, enough to fill about 1 teaspoon of filling. Spoon 1 teaspoon of apple butter filling into the cupcakes. Top with frosting. Add a dollop of apple butter to top of cupcake and swirl.
Optional: Add 2/3 cup finely chopped walnuts to apple butter filling.