In a large microwave safe bowl, place your butter.
Microwave for 15-second increments (stirring between) until butter is completely melted (if it's very hot, allow it to cool a few minutes, otherwise you'll melt your sugar)
Stir in sugar.
Add eggs and vanilla extract. Stir well.
In a separate bowl, whisk together your flour, cornstarch, baking powder, baking soda, and salt.
Gradually stir the dry ingredients into the wet, until completely combined.
Stir in sprinkles and nonpareils.
Transfer bowl of dough to refrigerator and allow to chill for at least 30 minutes.
Preheat oven to 350F and line cookie sheet with parchment paper.
Once dough has chilled, scoop (and roll, if you want round, pretty cookies -- not required) into 2 Tbsp-sized balls.
Place at least 2 inches apart on cookie sheet and bake on 350F for 12 minutes (cookies will still seem slightly underdone -- this is fine and helps make them nice and chewy; they will continue to cook on the cookie sheet as they cool.
Allow cookies to cool completely on cookie sheet before enjoying.