In a medium bowl, whisk together flour, cornstarch, baking powder, and salt; set aside.
In a bowl fit with a mixer, cream butter, and sugar on high until light and fluffy; about 5 minutes.
Add eggs, one at a time, mixing until incorporated. Beat in vanilla and almond extract.
Slowly add dry ingredients into wet ingredients. Beat until incorporated.
Cover bowl and refrigerate for at least 4 hours or overnight.
Preheat oven to 350 degrees F. Grease baking sheets or line with parchment paper.
Dough will be sticky. Use a cookie scoop to scoop dough onto baking sheets or simply use a tablespoon to scoop dough. If scoop gets sticky, dip it into a bowl of powdered sugar between scoops. Space cookies 1 inch apart.
Bake in oven for 13-15 minutes or until edges are slightly golden.
Allow to cool. Transfer to a cooling rack.
In a medium bowl, combine powdered sugar, water, and almond extract. Whisk until smooth.
Use a spoon to add glaze to each cookie over a cookie rack. Top with sprinkles.
Store in an airtight container.