Preheat & Prep:

Preheat oven to 410 °F (210 °C).

Line two baking sheets with parchment paper or silicone mats.

Cream Butter & Sugars (3 min):

In a stand mixer (or large bowl), beat the cold, cubed butter, light brown sugar, and granulated sugar on medium speed until light and fluffy.

Add Eggs & Vanilla:

With mixer on low, add eggs one at a time, mixing until just combined.

Pour in vanilla extract and blend until uniform.

Combine Dry Ingredients:

In a separate bowl, whisk flour, cornstarch, baking soda, and salt.

Gradually add to the butter-sugar mixture on low speed, mixing only until no streaks of flour remain.

Fold in Sprinkles & Candies:

Turn off mixer and use a spatula to gently fold in ½ cup sprinkles, ½ cup M&M’s, and white chocolate chips (if using).

Portion Dough:

Scoop or roll dough into 8 large (baseball-sized) balls, spacing them 3 inches apart on your prepared sheets.

Press a few extra M&M’s and sprinkles on top of each dough ball for a bakery-style look.

Bake (9–10 min):

Bake on the middle rack until edges are lightly golden and centers appear just set.

Rotate pans halfway through for even color.

Cool & Enjoy:

Let cookies rest on the hot sheet for 5 minutes to finish baking and firm up.

Transfer to a wire rack. Serve warm, or cool completely and store in an airtight container for up to 5 days.