Combine dry ingredients. In a medium bowl, whisk together the flour, baking soda, baking powder, cornstarch and salt. Set aside. 2 ½ cups (300 g) all-purpose flour 1 teaspoon baking soda 1 teaspoon baking powder ½ teaspoon fine sea salt 1 teaspoon cornstarch
Combine butter and peanut butter. In a microwave safe bowl, melt the butter. Then whisk the peanut butter into the hot, melted butter until well combined. ½ cup (1 stick, 113 g) unsalted butter ¾ cup (200 g) smooth peanut butter
Add sugars. With a spatula or wooden spoon stir in the granulated sugar and brown sugar until well combined. ½ cup (100 g) granulated sugar 1 cup (213 g) packed light brown sugar
Add eggs and vanilla. Add the eggs and yolk, one at a time, stirring well after each addition. Add in the vanilla. 2 large eggs 1 large egg yolk 2 teaspoons vanilla extract
Add dry ingredients. Stir in the flour mixture. Fold in the peanut butter chips. Cover and refrigerate the dough for at least 2 hours, up to overnight. Let the dough sit at room temperature for several minutes just until ready to scoop. 1 ¼ cups peanut butter chips
Preheat oven and prepare pan. Position a rack in a center of the oven and preheat the oven to 350℉/180℃.
Scoop cookie dough. Using a medium cookie scoop, scoop the cookie dough into 2-tablespoon portions (about 40-50 g each). Flatten the cookie dough and place about 1 ½ - 2 teaspoons fluff in the middle and fold the cookie dough around the fluff so no fluff is peeking through. You can use two spoons to place the fluff in the middle or pipe the fluff into the center. Place the cookie dough balls onto the prepared baking sheets spaced about 2-inches apart. 1 cup marshmallow fluff
Bake and cool cookies. Bake for 10-12 minutes, or until golden brown. Remove from the oven and cool for 5 minutes before removing to wire racks to cool completely. I like to top each cookie with a few additional peanut butter chip as soon as they come out of the oven for picture perfect cookies. Cookies can be stored in an airtight container at room temperature for up to 3 days.