Preheat oven to 325°F.
In a large bowl or stand mixer bowl, beat room temperature butter and powdered sugar together with an electric mixer until light and fluffy (about 2-3 minutes). If using a stand mixer, use a paddle attachment.
Gradually mix in all-purpose flour. Scrape down the bowl. Mix again until smooth.
Add cornstarch and blend again. Once combined, scrape down the sides and bottom of the bowl and blend again.
Using a cookie press, cookie scoop or two spoons, drop rounded tablespoons of cookie dough onto ungreased cookie sheets. Leave space between each cookie, as they will spread slightly. If desired, add sprinkles on top.
Bake 15-20 minutes, or until the edges are just set and slightly brown. The full cookie should still appear pale.
Cool 3-5 minutes on the baking sheet, then immediately transfer the cookies to a cooling rack to cool completely. Store in an airtight container at room temperature until ready to serve.
Snowball Cookies or Pecan Shortbread Cookies : Instead of sprinkles, add 1 cup finely chopped pecans to the dough and stir well. Scoop and roll dough into walnut-sized balls and place 2 inches apart onto ungreased baking sheets. Bake 12-15 minutes. Remove from the oven, cool 5 minutes, then transfer to wire racks. Add ½ cup powdered sugar to a small bowl. Roll hot cookies in sugar to coat, then return to the wire racks to cool.
Lemon Coconut Shortbread Cookies : Instead of sprinkles, add 1 cup sweetened shredded coconut (finely chopped), 1 teaspoon lemon zest, 1 ½ tablespoons lemon juice, and 1 teaspoon coconut extract to the dough and stir well. Scoop and roll dough into walnut-sized balls and place 2 inches apart onto ungreased baking sheets. Bake 12-15 minutes. Remove from the oven, cool 5 minutes, then transfer to wire racks. Add ½ cup powdered sugar to a small bowl. Roll hot cookies in sugar to coat, then return to the wire racks to cool.
Jam Shortbread Cookies: Instead of sprinkles, create an indention the center of cookie press pressed or scooped out cookies. In small bowl, stir ¼ cup seedless jam until smooth and free of lumps. Add ¼-1/3 teaspoon jam in the center. Bake 15-20 minutes.