Beat the melted butter and sugars together in a stand mixer fitted with the paddle attachment until well mixed, about 2 minutes. Add the egg, egg yolk, and vanilla and mix well.
In a separate bowl stir together the flour, baking soda, and salt. Mix the wet ingredients into the dry until just incorporated.
Drop spoonfuls of peanut butter and fluff into your dough and stir once or twice to incorporate only slightly. You should still have separate pockets of peanut butter and fluff within your cookie dough. Do not over mix. Set the dough in the fridge to chill for at least 1 hour, but up to 24 hours.
When you are ready to bake your dough, preheat the oven to 350°F , and prep a cookie sheet, lining it with parchment paper. Using a cookie scoop, drop the dough onto your sheet, leaving 2 inches of space between each cookie. Bake for 12 minutes, or until the edges are turning golden brown, but the center is still gooey. Allow to cool completely before eating. Enjoy!