Preheat the oven to 350. Line a 9-inch square baking pan with parchment or foil, leaving an overhang on all sides. Spray with nonstick cooking spray.
In a medium bowl, whisk together flour, cornstarch, baking powder, baking soda and salt.
In a large bowl, use an electric mixer on medium-high speed to beat the butter, Greek yogurt (or cream cheese) and sugar until light and fluffy, about 3 minutes. Add the egg, egg yolk, and vanilla and beat until well-combined. Slowly beat in the flour mixture. Stir in the sprinkles with a rubber spatula until just combined.
Press the dough into the prepared pan.
Bake for 25 minutes, or until lightly golden brown on top. Place the pan on a wire rack and let cool completely.