Preheat your oven to 350°F (175°C). This ensures that your cookies bake evenly.
Line your baking sheet with parchment paper to prevent the cookies from sticking.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large mixing bowl, use an electric mixer to cream together the unsalted butter, creamy peanut butter, light brown sugar, and granulated sugar until the mixture is light and fluffy.
Add the eggs one at a time, mixing well after each addition. Then, stir in the vanilla extract until well combined.
Gradually add the dry ingredient mixture to the wet mixture, mixing just until combined. Be careful not to overmix!
Gently fold in the semisweet chocolate chips, peanut butter chips, and Reese's Pieces until evenly distributed throughout the dough.
Using a large cookie scoop or a 1/4 cup measuring cup, scoop generous portions of dough onto the prepared baking sheet, leaving ample space between each cookie to allow for spreading.
If using, sprinkle a pinch of Fleur de sel on top of each cookie dough ball.
Bake in the preheated oven for 12-15 minutes, or until the edges are golden but the centers are still soft.
Allow the cookies to cool on the baking sheet for about 5 minutes before transferring them to a cooling rack to cool completely. Enjoy your colossal cookies with a glass of milk or your favorite beverage!
