Combine butter and sugar in the bowl of a stand mixer (or in a large bowl using an electric mixer) and beat until light and creamy.

Stir in egg, egg white, and vanilla extract until thoroughly combined.

In a separate bowl, whisk together cake flour, cornstarch, baking powder, and salt.

With mixer on low-speed, gradually add dry ingredients to wet until completely combined.

Form dough into a 1” thick disk and cover tightly with plastic wrap. Refrigerate for 1 hour.

Once dough has chilled, preheat oven to 350F (175C) and line baking sheets with parchment paper.

Remove dough from refrigerator. Transfer to a clean, lightly floured surface and use a rolling pin to roll dough to be between ¼-½” (about 1 cm) thick (if too is too sticky, return to refrigerator, if it is too difficult to roll and/or is cracking, let sit at room temperature for a bit before trying to roll again).

Use a 2” (5cm) round cookie cutter to cut out cookies as close together as possible and place on prepared baking sheet, spacing cookies at least 2” (5cm) apart.

Bake in 350F (175C) oven for 8-10 minutes. Allow cookies to cool on baking sheet for at least 5-10 minutes before carefully removing to cooling rack to cool completely Note: over-baked cookies are dry, crumbly cookies. Yours should be very pale coming out of the oven (not golden-brown), even the bottoms should be pale and not browned.

Once cookies have cooled completely, decorate with frosting.

In a large bowl (or stand mixer) using an electric mixer, beat butter until smooth and creamy. Add about half the powdered sugar, corn syrup, vanilla extract, and salt and stir until smooth and thoroughly combined, then gradually stir in remaining powdered sugar. If using food coloring, stir it in at the end adding a small bit at a time until desired color is reached.

Spread or pipe frosting evenly over cookies (I use a ½” open tip) and decorate with sprinkles, if desired.