Preheat the oven to 325°F. Grease cookie sheets or line them with parchment paper.
Sift together the flour, baking soda, and salt; set aside.
In a large bowl, beat the brown sugar, melted butter, white sugar, and peanut butter with an electric mixer until smooth.
Add the egg, egg yolk, and vanilla extract, and beat until the mixture is light and creamy.
Gradually stir in the flour mixture until the dough is just combined.
Fold in the semisweet chocolate chips, peanut butter chips, and rainbow sprinkles until evenly distributed.
Using a large cookie scoop, place cookie dough on the prepared pan. Add a couple of sprinkles to the top and lightly press them down into the dough
Bake in the preheated oven until the edges are golden, about 14-16 minutes. Remove them from the oven when the center feels slightly undone. They will continue baking on the sheet out of the oven.
Cool on the cookie sheet for 10 before transferring the cookies to a wire rack to cool completely.