• In a large mixing bowl, cream together butter and sugar for 4 minutes or until light and fluffy. Scrapes the sides of the bowl halfway through mixing.
• Add egg, egg yolk, and vanilla extract for 1 minute longer.
• Fold in flour, baking powder, baking soda, and salt. Don't overmix it.
• Chill the dough. Wrap tightly and refrigerate for at least 1 hour.
• Preheat oven to 350 degrees. Line light-colored baking sheets with parchment paper. Roll the chilled sugar cookie dough into balls and place on baking sheet.
• Bake for 9-12 minutes or until lightly golden on the edges but still soft in the center. DO NOT OVERAKE. These should be soft and chewy. As soon as you remove from the oven, sprinkle with sparkling sugar. Let cool.

Make Cream Cheese Frosting:

• In a mixing bowl with a hand mixer or with a stand mixer, beat butter and cream cheese for 3-4 minutes until light and fluffy. Add powdered sugar and vanilla and beat for 2 minutes longer.

Assemble Cookies:

• Spread a generous dollop of frosting on the flat side of one cookie. Top with another cookie, flat side down, to create a perfect sandwich. Press gently until the frosting reaches the edges.
• Tips for frosting sandwich cookies: Use a Piping Bag: For extra neatness, spoon the frosting into a piping bag (or zip-top bag with a corner snipped off) and pipe a swirl onto the center of the cookie’s flat side. Leave a Border: Don’t spread frosting all the way to the cookie’s edge initially. When you top it with the second cookie and press gently, the frosting will naturally push out to the edges