Whisk the dry ingredients. In a medium bowl, whisk together the flour, cornstarch, baking soda, baking powder and salt. Set aside.
Cream the butter and sugars. In a large bowl, beat the softened butter, granulated sugar and brown sugar together for 2–3 minutes, until pale, fluffy and lighter in colour.
Add the eggs and extracts. Beat in the egg, then the extra yolk, then the vanilla and almond extract, mixing just until combined. Scrape down the sides of the bowl once.
Combine. Add the dry ingredients to the wet and mix on low (or stir by hand) just until no dry flour streaks remain.
Fold in the sprinkles. Gently fold in the 1/3 cup of sprinkles with a spatula.
Chill the dough for 30 minutes. Cover the bowl and refrigerate.
Scoop. Scoop the dough into 2-tablespoon balls and place them 2 inches apart on a parchment-lined baking sheet. Press a few extra sprinkles onto the tops.
Bake at 350°F (175°C) for 9–11 minutes. Bake until the edges look set and very lightly golden, while the centres are still pale and look slightly underdone.
Rest on the tray for 10 minutes. Leave the cookies on the hot baking sheet for a full 10 minutes, then move to a wire rack.