Preheat the oven to 300°F (150°C), or 130°C for a fan oven. Line 2 cookie sheets with parchment paper or a silicone mat and set aside.
Add the butter and powdered sugar to the bowl of a stand mixer fitted with the paddle attachment and whip on high speed until light and fluffy, scraping down the sides as needed, about 5 minutes. The butter will be very pale.
Add the vanilla extract and mix again.
Add the flour and cornstarch, then mix until smooth.
Using a small cookie scoop, drop the cookie dough onto the prepared sheet 2 inches apart.
Using a fork, gently press the dough balls down slightly. If the fork sticks to the dough, dip it in powdered sugar before pressing on the cookies. Sprinkle the sprinkles on if using, then bake in the oven for 15-18 minutes, or until the cookies are puffed and lightly golden.
Allow to cool for 5 minutes before transferring to a wire rack to cool completely.
Notes
Whipping the butter: It's best to use a stand mixer for this recipe, but a hand-held electric mixer works too. Keep whipping the butter and sugar until it's a very pale color and is fluffy and airy.
No chilling needed: I've designed this cookie recipe to not require any chilling time. The only reason you may need to chill the dough is if it is particularly hot in your kitchen. At room temperature, the dough will be ready to bake as soon as it is mixed.
Pressing with a fork: If your fork is sticking to the cookie dough, dip the tines in powdered sugar before gently pressing down on the cookies. Don't press down too hard, go only enough to make a mark in the dough, keeping the cookies thick.
Cooling: Allow your whipped shortbread cookies to cool on the pan for about 5 minutes before removing them to a wire rack to cool completely. These cookies are very delicate when they are warm.
Added flavorings: You can add a small amount of your favorite baking extract to this recipe to create new cookie flavors! Try almond, mint, lemon, or orange extract.