Preheat the oven to 350°F. Line two baking sheets with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment (or using a large bowl and a hand mixer), beat the butter, brown sugar, and granulated sugar until light and fluffy, about 2 to 3 minutes.
Add the egg, egg yolk, and vanilla. Beat until fully combined, scraping down the sides of the bowl as needed.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the butter mixture, mixing just until combined. Do not overmix.
Fold in 2 cups of the Fruity Pebbles cereal and the mini marshmallows until evenly distributed. Folding by hand helps keep the cereal from breaking apart.b
Using a 3-tablespoon cookie scoop, scoop the dough and gently roll it between your palms to form smooth balls. Roll each dough ball in the remaining 1 cup Fruity Pebbles cereal, pressing lightly so the cereal adheres. Place the cookies on the prepared baking sheets about 2 inches apart.
Bake for 13 to 14 minutes, or until the edges are just beginning to turn light golden brown. (The melted marshmallows may brown before the cookies do.)
If any marshmallows ooze out during baking, use the back of a spoon to gently nudge them back toward the cookies while they are still warm. Let the cookies cool completely on the baking sheet before serving.