Preheat your oven to 350°F.
In the bowl of your stand mixer, beat the softened butter and sugar until light and fluffy, approximately 2–3 minutes.
Add the egg and vanilla extract and mix until combined.
In a separate bowl, sift together the flour, cornstarch, baking soda, baking powder, and salt.
Slowly add the dry ingredients to the wet ingredients while mixing on low speed.
Mix until everything is incorporated, then fold in the Halloween sprinkles.
Line a baking sheet with parchment paper and scoop 1½ tablespoons of dough into balls.
Place the cookie dough balls approximately 2 inches apart on the prepared baking sheet.
Bake for 10–12 minutes, until the edges are set and the centers are still slightly soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.
Store in an airtight container for up to 5 days or freeze for up to 3 months.