Begin by preheating your oven to 300 degrees and lining flat cookie sheets with parchment paper. In a large mixing bowl, cream together 1/2 cup cold butter and 1/2 cup Crisco for about 40 seconds until just mixed—avoid overmixing at this stage.
Add 2 cups white sugar and blend until just combined. Incorporate 1 teaspoon baking soda, 1/8 teaspoon salt, and 1 teaspoon cream of tartar into the mixture.
Continue by adding 1 teaspoon vanilla and the yolks from 3 large eggs, mixing until well combined. Finally, add 1 3/4 cups all-purpose flour and blend until the dough just comes together without overmixing.
Roll the dough into balls slightly larger than a hazelnut but smaller than a walnut. Roll each ball in red and green decorating sugar to coat evenly, then place them on the parchment-lined sheets about two inches apart.
Bake for 12 to 14 minutes, with 12 minutes being the recommended time for ideal results. Watch carefully to monitor the cookies develop golden, light bottoms without browning on the edges.
The cookies will appear puffy when removed from the oven, which is the desired result—they will continue to set as they cool.