Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.
Cream together butter, peanut butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the egg and vanilla extract until well incorporated.
In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
Gradually add dry ingredients to the wet ingredients, mixing until just combined.
Roll dough into 24 uniform balls and place on baking sheets, flattening slightly with a fork in a crosshatch pattern.
Bake for 8 to 10 minutes or until edges are lightly golden; cool completely on a wire rack.
Prepare ganache by heating heavy cream until simmering, pouring over chopped chocolate, and stirring until smooth after 2 minutes of resting.
Spread a layer of thickened chocolate ganache on the bottom side of 12 cooled cookies.
Spread marshmallow fluff on the bottom side of the remaining 12 cookies and press the halves together to form sandwiches.