Add the butter and sugar to the bowl of a stand mixer. Using the paddle attachment, cream them together.
Mix in the sour cream and vanilla extract until evenly incorporated.
Mix in half the egg whites until evenly incorporated, repeat with the second half.
In a medium sized bowl, whisk together the flour, baking powder and salt.
In a separate small bowl, stir together the apple butter and milk until smooth.
Add half of the dry ingredients to the batter and mix until well combined. Add the apple butter mixture and mix until well combined. Add in the rest of the dry ingredients and again mix until everything's incorporated and the batter is smooth.
Line a cupcake pan and scoop the batter into the liners, filling each about half way full. Bake at 350 degrees for 17-19 minutes, or until a toothpick stuck in the center comes out clean.
Remove the cooked cupcakes from the oven to cool. After resting in the pan for a few minutes, transfer the cupcakes to wire racks to finish cooling completely.