Line each baking sheet with a piece of parchment paper and set aside. Combine the dry ingredients using a dough hook or whisk and set aside.
Cream together the butter and sugar until completely smooth using a stand or hand mixer and then add the egg and vanilla, mixing after each addition. When this mixture is smooth, add the dry ingredients in one or a few batches. Add in a few batches if you are using a hand mixer or a small stand mixer.
When combined, turn the dough out onto plastic wrap and place the dough ball in the fridge for 45 minutes (or up to 3 days). If you used cold butter, you can skip this step. But anytime your prepared dough has been out of the fridge for more than 20 minutes (aside from made ahead dough - see below) place the cookie dough back into the fridge. For best results, cookie dough to be cool yet easy to roll anytime you’re rolling and cutting.
Preheat oven to 350 degrees and transfer the dough ball to a lightly floured surface. Use a rolling pin to roll out the dough until it is approximately ⅛ to ¼” thick. Place snowflake cookies on prepared trays and, if the dough has been sitting out for more than 20 minutes, place the tray in the fridge for 10 minutes before baking.
Bake cookies for 8-12 minutes or until the bottom edges are just starting to turn golden brown.
Let the cookies cool all the way on the tray while you make the homemade buttercream frosting.