In a small mixing bowl, whisk together the flour, salt, baking powder, and baking soda until combined.
1½ cups all-purpose flour, ½ teaspoon sea salt or kosher salt, ¼ teaspoon baking powder, ¼ teaspoon baking soda
In a separate larger mixing bowl, or you can use a stand mixer, cream together the butter and sugar for 2-3 minutes or until light and fluffy. Start on low speed and increase speed as needed.
1/2 cup (1 stick) unsalted butter, 3/4 cup granulated sugar
Scrape down the sides and bottom of the bowl and add in the egg and vanilla extract. Beat until well combined and incorporated.
1 large egg, 1 teaspoon vanilla extract
Scrape down the sides and bottom of the bowl again and dump in the dry ingredients. Mix on low speed just until combined and no flour streaks remain in the dough.
Stir in the rainbow sprinkles with a wooden spoon or spatula.
1/3 cup rainbow jimmies sprinkles
Scoop the cookie dough, using a medium cookie scoop (1½ tablespoons), and roll it into a ball. Place them onto a plate.
Chill the dough for 1 hour in the fridge.
When ready, preheat the oven to 350℉. Line two cookie sheets with parchment paper.
Place some additional sprinkles into a small bowl. Take one cookie dough ball at a time and gently press the top of it into the sprinkles. Place 12 cookies per cookie sheet with the sprinkles facing up.
Bake one sheet at a time for 9-11 minutes.
Allow the cookies to cool on the warm cookie sheet for about 5 minutes before removing them to a cooling rack to cool completely.