Heat oven to 350°F. Line baking sheets with parchment paper.
In medium bowl, whisk together flour, baking soda, baking powder, and salt.2 cups all-purpose flour,1 teaspoon baking soda,1/2 teaspoon baking powder,1/2 teaspoon salt/
In large bowl, beat butter, peanut butter, granulated white sugar, and brown sugar until light and fluffy, about 3 minutes.1/2 cup unsalted butter,3/4 cup creamy peanut butter,3/4 cup granulated sugar,3/4 cup packed light brown sugar.
Add eggs and vanilla, beating until well combined.2 large eggs,1 teaspoon vanilla extract.
Gradually mix dry ingredients into wet ingredients until just combined.
Using a large cookie scoop, scoop cookie up, then flatten each into flat disk.
Place a dollop of marshmallow fluff in the center of each disk (about the size of a marble). Don't overfill or it will leak out during baking. 3/4 cup marshmallow fluff.
Bring dough up around marshmallow fluff, completely covering and forming a ball.
Place on prepared baking sheets about 2 inches apart.
Bake 10-12 minutes until edges are brown. Watch carefully so marshmallow does not burn.
Let rest on baking sheet 10-15 minutes before serving.
Notes
PRO TIP: Make sure you completely seal the marshmallow fluff inside the dough! If there are any gaps, it will leak out during baking. A little leaking is okay and actually looks good, but you want most of it to stay inside.Also, don't skip the resting time, those centers are like hot lava right out of the oven! If you try to move them too soon, they'll fall apart!