Preheat oven to 350 degrees F. Line two baking sheets with parchment paper; set aside.
In a medium bowl, whisk together the flour, baking soda, and salt; set aside.
With an electric mixer on medium speed, cream together the butter, peanut butter, and both sugars until light and fluffy, about 3 minutes. Scrape down the sides of the bowl, add the eggs and vanilla extract, and beat until combined. Reduce the mixer speed to low and gradually add the flour, mixing until it has almost all been incorporated. Add the chocolate chips, peanut butter chips, and Reese's Pieces, and mix briefly (about 20 seconds) to ensure they are evenly distributed.
Roll a ¼ cup of dough into a ball and place on one of the baking sheets. Repeat with the rest of the dough - you should end up with about 18 cookies. With your fingers, press down on the balls of dough to flatten into disks, about ½-inch thick. Sprinkle with a pinch of Fleur de Sel. Bake the cookies for 12 to 15 minutes, just until the edges of the cookies are beginning to turn a light brown. Cool the cookies on the baking sheets. Store in an airtight container at room temperature for up to 5 days.