In a medium bowl, whisk together the all-purpose flour, baking soda, and cream of tartar. Set to the side.
Using your hand mixer or stand mixer fitted with a paddle attachment, beat the unsalted butter on medium-high speed until smooth and creamy.
Turn off your mixer and add in the powdered sugar. Blend it together with the butter over low speed until it's almost combined.
Scrape down the sides of the bowl, then on medium-high speed, add in the egg, vanilla extract, and almond extract until well blended.
Switching the mixer speed low, gradually add in the whisked dry mixture until fully incorporated. The cookie dough should start to form a ball and pull away from the sides.
Transfer the sugar cookie dough onto a clean work surface, then shape it into a ball and divide it into 2 sections. Roll out the first section of cookie dough to a ¼ inch thickness slab on a lightly floured sheet of parchment paper (or wax paper if you plan to freeze the dough for later). Place onto a baking sheet or jelly roll pan. Repeat with the second section, placing it on top of the first layer. Refrigerate the cookie dough slabs for a minimum of 1 hour or (my preference) overnight.
After chilling is complete, adjust your oven rack to the 2nd level position (just above center) and preheat your oven to 375ºF. Line 2 cookie sheets with parchment paper and set aside.
Flour your cookie cutters and cut out your shapes. Place each cut out shape about 1"- 1 ½" apart on the cookie sheet. If only using sanding sugar or sprinkles, now is the time to add them. Place any cut out cookies not yet baked into the refrigerator or freezer to chill while the first set bakes.
Bake the cookies for 7-8 minutes at 375ºF or until the edges are a light golden brown. Allow them to cool on the cookie sheet for about 5 minutes before transferring them with a spatula (or server) to a wire cooling rack. Let them cool completely before decorating them with the royal icing. If baking the cookies in advance, keep them stored in an airtight container with a slice of bread for up to 3 days. You can also freeze baked cookies for up to 3 months. Let them cool completely before placing into a well-sealed container or freezer bag.