Preheat the oven to 350 degrees F. Line a 9-inch by 13-inch baking pan with parchment paper, then spray with nonstick cooking spray.NOTE: letting the parchment paper overhand the sides of the pan makes it easier to lift the cooled bars out for cutting. In a medium bowl combine the dry ingredients: flour, baking soda, and salt. Whisk to mix fully. Set aside.
2 1/4 cups all purpose flour, 1 teaspoon baking soda, 1/2 teaspoon kosher salt
In a large bowl using a electric hand mixer, or in the bowl of a stand mixer fitted with paddle attachment, beat together the wet ingredients: butter, brown sugar, and sugar until light and fluffy (2-3 minutes).
1 cup unsalted butter, 3/4 cup brown sugar, 3/4 cup sugar
Add the eggs and vanilla extract and beat to combine. Add the flour mixture and beat to mix together. 2 large eggs, 1 teaspoon vanilla extract
Fold in the chocolate chips, peanut butter chips, and Reeses pieces by hand.
1 cup semi-sweet chocolate chips, 3/4 cup peanut butter baking chips, 1 1/4 cups Reese’s Pieces candies
Cover the bowl with plastic wrap and chill for at least 30 minutes.
Spread the dough into the prepared pan to an even thickness. Sprinkle on extra candies on top before baking (optional).
Bake in the preheated oven for 25 to 30 minutes, until the edges are golden brown but the center is still soft. A toothpick inserted into the center should come out clean.
Allow the cookie bars to cool in the pan on a wire rack. When cool slice into 24 bars and serve.