Combine the butter, brown sugar, and granulated sugar in the bowl of a stand mixer fitted with the paddle attachment (or a large mixing bowl with an electric hand mixer). Beat on medium-high speed for about 2 minutes to cream the ingredients together until smooth and creamy.
Add the vanilla extract and egg. Beat on medium speed until incorporated, scraping down the sides of the bowl as needed.
With the mixer on low speed, add the baking powder, baking soda, mixing just until combined. Gently fold in 1 ½ cups of Fruity Pebbles.
Cover the cookie dough with plastic wrap and chill it in the refrigerator for 30 minutes.
Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper or a silicone mat.
After chilling the dough for 30 minutes, scoop out 12 balls of cookie dough, rolling them in your hands and then flattening them slightly into discs. Dip the top of each cookie in the remaining 1/2 cup of Fruity Pebbles. Place cookies about 1 ½ inches apart on prepared baking sheet.
Bake cookies for 10 minutes. Remove from oven and let the cookies cool on the pan for a few minutes before transferring them to a cooling rack to cool completely. Enjoy!