Preheat your oven to 350°F (175°C).
Grease your muffin pan with cooking spray or butter.
In a large mixing bowl, combine melted butter, light brown sugar, granulated sugar, peanut butter, and vanilla extract. Beat until smooth.
Add the egg and mix until fully incorporated.
In a separate bowl, whisk together flour, baking soda, and salt. Gradually add to wet ingredients, stirring until just combined.
Scoop a tablespoon of dough into each muffin cup, filling halfway.
Place a mini peanut butter cup in the center of each filled muffin cup, pressing down gently.
Bake for 10-12 minutes, until edges are golden and centers look set.
Cool for 5 minutes in the pan, then transfer to a wire rack to cool completely.