Preheat the oven to 350°F (175°C).
Prepare a muffin tin with liners or grease it lightly.
In a large bowl, combine the flour, cocoa powder, baking soda, salt, and sugar.
In another bowl, whisk together the peanut butter, milk, egg, and vanilla extract until smooth.
Gradually add the wet mixture to the dry ingredients, stirring until just combined.
Spoon the batter into the prepared muffin tin and fill each cup about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for about 5 minutes before transferring them to a wire rack.
