1. Preheat the oven to 350°F (175°C). Line a muffin tin with cupcake liners.
2. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt.
3. In a large bowl, beat together the granulated sugar, brown sugar, eggs, buttermilk, vegetable oil, and vanilla extract until smooth.
4. Gradually add the dry ingredients to the wet ingredients, mixing until combined.
5. Add the boiling water to the batter and mix until smooth. The batter will be thin, but this is normal.
6. Spoon the batter into the prepared muffin tin, filling each liner about halfway.
7. Drop a spoonful of peanut butter (about 1 teaspoon) into the center of each cupcake. Top with more batter to cover the peanut butter, filling each liner about 3/4 full.
8. Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean (except for the gooey peanut butter center). Let the cupcakes cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

For the Peanut Butter Filling:

1. In a small bowl, mix together the peanut butter and powdered sugar until smooth. Set aside.

For the Peanut Butter Frosting:

1. In a medium bowl, beat together the softened butter and peanut butter until smooth and creamy.
2. Gradually add the powdered sugar, 1 cup at a time, and mix until fully incorporated.
3. Add the vanilla extract and milk, and beat until the frosting reaches your desired consistency. If it’s too thick, add a little more milk; if too thin, add more powdered sugar.

Assembling the Cupcakes:

1. Once the cupcakes have cooled completely, use a small knife to create a small hole in the center of each cupcake.
2. Spoon the peanut butter filling into each hole, then top with a swirl of peanut butter frosting.
3. Optionally, drizzle with melted chocolate or top with chopped peanuts for extra flavor and decoration.