Pre-heat oven to 350 degrees and line an 8 inch (or 9 inch) square baking pan with enough aluminum foil so there is an over-hang.

In a medium bowl, sift together the gluten-free all-purpose flour, xanthan gum, salt, baking powder, baking soda and cornstarch. Stir and set aside.

Cream together 1/2 C butter and the granulated sugar until light and fluffy. Scrape down the bowl and beat in the egg and vanilla extract. Add the dry ingredients and mix until just combined, scraping the bowl as needed.

Transfer the dough to the prepared pan and spread evenly in the pan. The dough will be sticky so it might help to press the dough in with well-floured hands.

Bake at 350 degrees for 22-25 minutes.

Cool in the pan on a wire rack. Do not frost until completely cool.

While the sugar cookie bars are cooling, make the frosting. To make the frosting, Beat together 1/2 cup butter and confectioners' sugar on low. Increase speed to medium and beat for 3 minutes. Add vanilla and whipping cream and food coloring (if using) and beat for another minute. For the orange color I used 12 drops of yellow and 6 drops of red food coloring.

Use the aluminum foil to lift the sugar cookie bars from the pan. Evenly spread frosting over the completely cooled bars and top with sprinkles. Makes 16 bars. Enjoy!