Line two cupcake pans with cupcake liners and preheat the oven to 350 degrees.

In a medium sized bowl, beat butter on medium speed with an electric mixer until smooth and creamy. Add sugar and vanilla, then mix well. Add eggs one at a time, mixing well after each.

In another bowl, combine flour, baking powder, and salt. Add half of the flour mixture and half of the buttermilk to the sugar mixture. Mix until just combined. Repeat with the second half of the flour and buttermilk and mix well.

Fill the bottom of each cupcake liner with a little bit of cake batter, just enough to cover the bottom. Add a cookie butter cup, then top with more cake batter. The cupcake liners should be about 2/3 full.

Bake for 20 minutes or until lightly golden brown on top. Let the cupcakes cool completely before frosting.

In the bowl of a stand mixer, cream butter on medium speed until smooth and creamy. Add cookie butter, vanilla, and milk. Mix well. Add half of the powdered sugar, mix on low speed until just combined, and then medium speed until completely combined. Repeat with the second half of the powdered sugar. Spread frosting on top of cupcakes and top with half of a cookie butter cup.