Preheat your oven to 350°F and fill a cupcake pan with liners. Set the pan aside.
Combine the salted butter and granulated sugar in a large mixing bowl. Using an electric hand mixer, cream the butter and sugar together for about 2 minutes, or until the sugar is completely dissolved.
Whisk in the eggs until they are thoroughly beaten and fully incorporated into the creamed sugar. Add the vanilla, milk, and ½ cup of peanut butter. Mix until combined.
Reduce the speed of your mixer and gently whisk in the baking soda, baking powder, and flour. Make sure no clumps of dry flour remain in your cupcake batter.
Divide the batter evenly among the 12 cupcake liners and bake for 20 minutes, or until you can cleanly remove a toothpick from your cupcakes.
Remove the cupcakes from the oven and poke a hole in the middle of each cupcake that is about ¾ inch in diameter. Fill the holes with the other ½ cup of peanut butter.
Allow your cupcakes to cool while you prepare the frosting by whisking together the powdered sugar, unsalted butter, milk, and peanut butter until smooth and creamy.
Add your frosting to a piping bag and frost each cupcake however you’d like.
Serve and enjoy!