With a countertop stand mixer and whisk attachment, whisk the room temperature butter and powdered sugar together for 3-4 minutes until fluffy, scraping down the sides of the bowl several times.
Sift together the flour and cornstarch. Carefully incorporate it into the butter and sugar mixture until just combined, about one minute on low.
Cover and refrigerate for about 1 ½ hours.
Preheat the oven to 300°F and line a baking sheet with parchment paper.
With your hands, roll one inch cookie dough balls and place them several inches apart on the prepared baking sheet.
Take a fork and carefully press down on each cookie ball. Add sprinkles of choice BEFORE baking to the top of each cookie.
Bake the cookies for 18 minutes. Remove to a cooling rack to cool.
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