Combine melted, cooled butter with sugar and stir until combined.

Add eggs and vanilla extract, stir well.

In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt until thoroughly combined.

Gradually stir dry ingredients into wet until completely combined.

Stir in sprinkles until well-distributed through the dough.

Cover bowl with plastic wrap and chill in the refrigerator for at least 30 minutes and up to 5 days (dough chilled longer than a few hours may need to sit at room temperature for a bit to become soft enough to scoop easily).

When nearly ready to bake, preheat oven to 350F (175C) and line a baking sheet with parchment paper.

Scoop dough by rounded 1 ½ Tablespoon-sized scoops and drop onto baking sheet (or roll the dough into smooth balls between your palms for rounder, more uniform cookies) spacing cookies at least 2” (5cm) apart.

Transfer baking sheet to 350F (175C) preheated oven and bake cookies for 10-12 minutes. Cookies may look slightly underdone when they come out of the oven, this is fine and makes them nice and chewy as they continue to cook on the baking sheet as they cool. Let them cool completely on the baking sheet before removing as they’re fragile when warm!