Preheat oven to 350°F and line two baking sheets with parchment paper.
In a bowl, whisk together flour, baking powder, baking soda, and salt; set aside.
In a separate bowl, beat butter and both sugars with a hand mixer until light and fluffy, 1-2 minutes.
Add the egg and vanilla extract to the butter mixture and mix until well combined, scraping bowl sides as needed.
On low speed, gradually add the dry flour mixture and mix until just combined.
Gently fold in the M&M's and white chocolate chips.
Pour the sprinkles into a shallow bowl.
Scoop cookie dough balls (1 1/2 to 2 1/2 tablespoons) and gently press the top of each ball into the sprinkles.
Place dough balls onto prepared baking sheets, spacing 1-2 inches apart, mounded higher for thicker cookies.
Optionally chill dough balls in the refrigerator for 20 minutes for thicker cookies.
Bake cookies for 8-11 minutes until edges are set but centers remain slightly soft.
Remove from oven and, while warm, optionally press extra M&M's or chocolate chips on top.
Transfer baking sheets to wire racks and allow cookies to cool completely.