1. Prepare the Cookie Dough

• In a large bowl, cream together the softened butter and sugar for 2–3 minutes until smooth and fluffy.
• Add the egg and vanilla extract. Mix until well combined.
• In another bowl, whisk together the flour, cornstarch, baking powder, and salt.
• Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Avoid overmixing.

2. Shape and Roll in Sprinkles

• Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
• Scoop out 1-tablespoon portions of dough and roll into smooth balls.
• Roll each ball in Christmas-colored sprinkles to fully coat the surface.
• Place the dough balls on the prepared baking sheets, spacing them about 2 inches apart.

3. Bake the Cookies

• Bake in the preheated oven for 10–12 minutes until the edges are set but the centers remain soft.
• Let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.

4. Make the Buttercream Frosting (Optional)

• In a small bowl, whip the softened butter until smooth. Gradually add powdered sugar, mixing until incorporated.
• Stir in the vanilla extract and salt. Add milk, 1 tablespoon at a time, until you reach a smooth, spreadable consistency.
• Divide the frosting into two bowls and tint one red and the other green with gel food colorings.

5. Assemble the Stacked Cookies

• Spread a layer of buttercream frosting on the bottom of one cookie and stack a second cookie on top to form a sandwich.
• Pipe decorative swirls of frosting on top for extra flair and sprinkle with more festive decorations.

6. Serve and Store

• Allow the frosting to set before serving. Store the cookies in an airtight container at room temperature for up to 1 week or freeze for longer storage.