Preheat oven to 400 degrees. Line muffin tin with papers.
Add flour, sugar, chocolate chunks, cocoa powder, baking soda, and espresso powder to a large bowl. Whisk to combine.
Add sour cream, milk, vegetable oil, egg, and vanilla to a medium bowl and whisk well to combine.
Pour wet ingredients into dry ingredients and stir until combined.
In a small bowl, stir together the peanut butter and powdered sugar with a fork until well combined and smooth.
Spoon muffin batter into muffin liners, filling just 1/3 of the way full. Top with a small spoonful of peanut butter mixture. Top with more muffin batter to fill 3/4 of the way full.
Bake for 15 minutes or until a tester comes out mostly clean, with just a few moist crumbs.
Cool completely. Drizzle with additional melted peanut butter before serving, if desired.