Preheat oven to 350F. Line two baking sheets with parchment paper. Set aside.
In a medium bowl, whisk together the flour, baking soda, baking powder and salt.
2 ¾ cups all purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ¾ teaspoon fine sea salt
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, granulated sugar and brown sugar and beat until light and fluffy, 3 minutes.
1 cup unsalted butter, room temperature, 1 cup granulated sugar, ½ cup light brown sugar, packed
Scrape the bottom and sides of the bowl with a spatula.
Beat in the egg, egg yolk, vanilla and mix until well combined.
1 large egg, 1 large egg yolk, 2 teaspoons pure vanilla extract, or paste
Add the dry ingredients and mix on low until just combined. The dough will be thick.
Remove the bowl from the mixer and fold in the sprinkles.
½ cup rainbow sprinkles (jimmies)
Place the ⅓ cup of sugar into a bowl. Using a 2 tablespoon ice cream scoop, scoop out the dough and form into balls.
⅓ cup granulated sugar
Roll the dough balls in the sugar to coat and place two inches apart on the cookies sheets. Use the bottom of a glass or measuring cup to flatten the dough balls just slightly.
Bake for 10-12 minutes, until the edges start to turn a light brown and the top begins to crack.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a cooling rack to cool completely.