In a large bowl, beat the softened butter and sugar. Add the egg yolks, vanilla, and salt, and combine. Add the flour and beat until it comes together to form a rough ball. If the dough is too soft, add flour 1 tablespoon at a time until it becomes easy to shape into a ball.
Flatten the dough into a thick round disk and wrap with plastic wrap. Refrigerate for 30 minutes to 1 hour or overnight.
Preheat the oven to 350°F.
Generously flour a clean work surface. Place the round of dough on the floured work space and use a rolling pin to roll out the dough. Keep the dough thick, at just under ½ inch thickness. If the dough is too cold, let it sit out for 10-15 minutes to warm up and soften slightly.
Use cookie cutters to cut out shapes. Place the cut out cookies on a baking sheet, leaving a little room between the cookies. Decorate with sanding sugar if desired.
To make homemade sanding sugar, add ½ cup granulated sugar to a bowl along with 5 to 10 drops of food coloring (you can also use food gel), and combine, mashing with the back of a spoon as needed to break up any concentrated clumps of the food dye.
Bake for 8-10 minutes. Do not allow to brown. Remove and let cool before removing the cookies from the baking sheet.
I recommend doing a test cookie before baking a whole batch to ensure that you have added the right proportion of butter and flour and the dough is the right temperature to prevent the cookies from spreading in the oven. That way, you can add more flour if needed or chill the cookies to reduce spreading.If the cookies did spread further than you would like, you can address this by quickly taking the same cookie cutters to re-cut the cookies after taking them out of the oven. You’ll need to do this when they are fresh out of the oven and still warm. This will give them nice, sharp, crisp edges.
Notes
Refrigerating the Dough: Shaping the dough into a flattened rectangle makes it easier to roll out the dough once it has had time to chill.
Rolling Out the Dough: If the dough is too cold to roll out, let it sit for 10-15 minutes at room temperature, and it will soften enough to roll out. You don't want the dough to be too soft, or the cookies will spread when baked in the oven.
Transferring Cookies to a Baking Sheet: To minimize handling of the dough, once a cookie has been cut, pick up the cookie cutter while keeping the dough inside. Then hold it over the baking sheet and gently push the cut cookie out of the cookie cutter onto the baking sheet.
Decorating with Sanding Sugar: Using homemade sanding sugar before baking the cookies allows the sugar to slightly melt on the cookies for a cohesive look, where the sugar sticks to the cookies. If you wait until after the cookies have baked, it's much more difficult to get the sugar to stick.