Preheat oven to 350°F (175°C). Line a cupcake tray with liners.
In a bowl, whisk together flour, cocoa powder, baking soda, and salt.
In another bowl, beat melted butter, sugars, buttermilk, eggs, and vanilla until smooth.
Gradually mix in dry ingredients until just combined.
Fill cupcake liners halfway, then drop a spoonful of peanut butter into each and swirl.
Drizzle melted chocolate over the batter.
Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean.
Cool before serving.