Preheat the oven to 350°F. Line two baking pan with parchment paper or silicone baking mat.
Add the flour, cornstarch, baking powder, and sea salt into a medium bowl and stir well. Set aside.
Beat the butter and sugar together in a mixing bowl until light and fluffy - about 4 to 5 minutes.
Add in the egg, yolk, and vanilla extract. Beat well for about 4 minutes.
Pour the flour mixture into the butter mixture and gently mix just until there are no more streaks of flour. Do not over mix.
Fold in the sprinkles and gently stir until fully mixed in.
Create balls of cookie dough that about roughly 1 ½ tablespoons big.
Chill the balls in the freezer for about 5 - 10 minutes. This step is optional, but I do recommend it.
Spread the balls 2-inches apart on your baking sheets. Bake for 8 - 10 minutes, or until the edges just start to gently color. The tops will look underbaked.
Allow the cookies to cool on the baking sheet for 2 minutes before you transfer to a wire rack.
Notes
It is important to not overmix your dough otherwise the cookies will not be soft and tender. Taking out the cookies while underbaked will help keep them soft and chewy. The cookies will continue to bake while on the hot baking sheet.