In a medium bowl, whisk together the flour, cornstarch, baking soda, baking powder, and salt. Set aside.

Add the butter to a saucepan over medium heat. Cook, stirring often, until the butter becomes golden brown and develops toasted brown bits at the bottom of the pan. Remove from the heat and let cool for 10–15 minutes.

In a large mixing bowl, whisk together the browned butter, brown sugar, and granulated sugar until smooth and combined.

Add the egg, egg yolk, and vanilla extract. Mix until fully incorporated.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix the dough.

Gently fold in the Fruity Pebbles cereal and white chocolate chips using a spatula.

Scoop the cookie dough into large 2-tablespoon-sized balls and place them on a parchment-lined baking sheet. For thicker cookies, chill the dough in the refrigerator for at least 30 minutes.

Preheat the oven to 350°F. Line a baking sheet with parchment paper or a silicone baking mat.

Arrange the cookie dough balls at least 2 inches apart on the prepared baking sheet. If desired, press a few extra Fruity Pebbles and white chocolate chips on top of each cookie dough ball before baking.

Bake for 10–12 minutes, or until the edges are lightly golden and the centers still look slightly soft.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. The cookies will continue to set as they cool.