Preheat your oven to 350°F and line a baking sheet with parchment paper.
Cream together the softened butter and sugar until light and fluffy, about 2-3 minutes.
Add the egg, egg yolk, and vanilla extract, mixing until smooth and fully incorporated.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Add the dry ingredients to the butter mixture and mix just until combined. Fold in the white chocolate chips and 1 cup of Fruity Pebbles cereal.
Using a 1-ounce cookie scoop, portion the dough. Roll each dough ball generously in 1 to 1½ cups of Fruity Pebbles, gently pressing the cereal into the outside to ensure good coverage.
Shape each cookie dough into a puck shape, then freeze until firm.
Bake directly from frozen at 350°F for 12 minutes.
As soon as the cookies come out of the oven, sprinkle a few more Fruity Pebbles over the tops and gently press them in if needed.
Finish by giving each cookie a light smash with a spatula for that thick, bakery-style look.