In a large bowl, combine butter and sugar. Using an electric mixer fitted with the paddle attachment or a wooden spoon by hand, cream together butter and sugar until smooth.

Add the egg and vanilla, and whisk to combine.

Add your flour, cornstarch, baking powder, and salt. Fold together until you have a streak free cookie dough. The cookie dough should be soft, but shouldn't stick to your hands. If it's too sticky to work with, pop it into the fridge for 30 to 60 minutes or until firmed up enough to handle.

Scoop a 1.5 tablespoon sized portion of dough and roll into a smooth ball. Roll the ball of dough around in your choice of colored sprinkles to completely coat. Place the sprinkle covered ball of dough on your parchment lined baking sheet and flatten into a ¼ inch thick disk.

Take another piece of dough (about half the size of the sprinkle covered dough) and press into a small disk shape that's smaller than the disk of sprinkle dough. Lay the plain disk of dough over the sprinkle covered dough and gently press to stick it on. See above for visual.

Repeat the process of stacking sprinkle covered dough with plain dough. Make sure to leave about 1 inch of space between each cookie on your baking sheet.

Bake the stacked cookies in the center of your preheated 350°F oven for 12 to 15 minutes, or until puffed and lightly browned along the bottom edges.

Cool the cookies for 5 minutes on the baking sheet, then carefully transfer onto a wire rack and set aside to cool completely to room temperature.

In a large bowl, combine butter, sugar, and salt. Using an electric mixer fitted with the paddle attachment or a wooden spoon by hand, mix together until the sugar is dissolved and the mixture is smooth and lightened in color.

Add vanilla and 1 tablespoon of milk, and whisk together until combined and fluffy. If needed, gradually whisk in more milk until you reach your desired consistency.

To create the designs I made for these cookies, divide the frosting into thirds. Leave ⅓ of the buttercream plain white, and color ⅓ red and ⅓ green.

Decorate the cookie tops with some with your favorite Christmassy designs - I just piped on some snowflakes, Christmas trees, and Santa hats! Before the frosting sets, sprinkle over some white sprinkles on the snowflakes for added sparkle and crunch. Sprinkle some rainbow sprinkles over the trees to resemble Christmas lights. Or you can also leave the cookies plain. Serve and enjoy!