These are my favorite Christmas cookies. My Mom makes them every year. She rolls the dough out quite thin, producing nice crispy cookies. These cookies freeze well, so make ahead of time and bring them out as you need. —Katheryn's Kitchen
Cream butter and sugar until light and fluffy, this will take 5-6 minutes
Beat egg and vanilla into butter and sugar mixture until well incorporated, you may need to scrap down the sides of the bowl once or twice.
Scrap down the bowl and add flour, baking soda and cream of tartar all at once. Slowly beat in until it comes together into a dough, this will take 3-4 minutes
Heat oven to 350°F. Prepare two baking sheets by lining them with parchment paper or silicon liners. Scrape the dough out of the mixing bowl onto a floured surface and knead for about 3 minutes or until the dough is nice and smooth. Cut in half and form into two disks. If the dough seems too soft and sticky, wrap the disks in plastic wrap and chill in the refrigerator for 15-20 minutes.
Roll out to about a 1/8 inch thickness on a well floured surface. Cut out cookies with your favorite Christmas cookie cutters. I like to sprinkle them with different colored sugar sprinkles. If you are going to ice them I would roll the dough out to 1/4 inch thickness instead of 1/8 inch so that they are a little sturdier and easier to handle. Our family prefers them thin and crispy with the sugar sprinkles, that's how my Mom has made them for 30 odd years. Use a spatula if necessary to move them onto the lined baking sheet. Place them about 1 inch apart. Knead the scraps into another disk and roll out again, keep doing so until there is no dough left to cut out cookies. Now sprinkle with coloured sugars and place in oven for 8-9 minutes, maybe a minute or 2 longer, depending on the thickness of the dough.
When finished, pull out of the oven, let cool for a few minutes then transfer the cookies onto cooling racks. When completely cool, store in a sealed container in the freezer for up to 3 months. Bring out as you need.