Whisk together the flour, cornstarch, confectioners' sugar, baking powder, and salt. Set aside.

In the bowl of a stand mixer, beat together the granulated sugar and the orange and lemon zest for 1 minute on medium-low speed, until the sugar turns light orange.

Add in the butter and beat until light and fluffy, about 2 minutes on medium-high speed.

Add in the egg and beat until incorporated. Mix in the orange extract.

Slowly mix in the dry ingredients just until incorporated.

Divide the dough into two portions, flattened into disks, and wrap each tightly with plastic wrap. Refrigerate at least 2 hours or overnight.

Preheat oven to 350 degrees (F), and line cookie sheets with parchment paper.

Allow the dough to soften at room temperature about 20 minutes before rolling out to 1/8" to 1/4" thickness on a lightly floured surface. Cut into snowflakes or whatever shapes you desire.

Place cutouts on the parchment lined baking sheets. Refrigerate the cutouts for at least 10 minutes before baking.

Bake for 10 to 14 minutes (depending on thickness), rotating baking sheet once halfway through the baking time. The edges should be slightly golden brown. Remove from oven and allow the cookies to cool for 5 minutes before transferring them to a wire rack. Cool completely before decorating.

Whisk together the confectioners' sugar and the orange and lemon juice until smooth. If icing is too thick, add additional juice, 1/4 teaspoon at a time, until desired consistency is reached.

Using a silicone pastry brush or the back of a spoon, smooth the icing over the cooled snowflake cookies. Garnish with sparkling sugar or white nonpareils if desired.

Allow the icing to set at least 2 hours, until firm. Store leftover cookies in an airtight container, layered between sheets of waxed paper, at room temperature for up to 3 days.

*You may use either all lemon or all orange in the recipe.