Add your softened butter and sugar to a mixing bowl and beat on medium-high speed for about two minutes. You're looking for a pale, fluffy texture rather than anything grainy or separated. This step builds the airy structure your dough needs.
Crack in your egg along with the vanilla and almond extract, then mix until fully combined. The almond extract is subtle but noticeable, giving the dough a slightly more complex flavor than a typical sugar cookie.
Whisk together your flour, baking soda, and cream of tartar in a separate bowl first. Add this dry mixture into your wet ingredients gradually, stirring just until a smooth dough comes together without overmixing it.
Shape the dough into a ball, cover it, and refrigerate for about an hour. This step firms everything up so your cookies keep a crisp, clean skull shape once you cut and bake them.
Once chilled, let the dough sit at room temperature for about ten minutes to soften slightly. Roll it out on a floured surface to about half an inch thick, then press your skull cutter firmly into the dough.
Arrange your cut cookies on a parchment-lined sheet with a little space between each one. Bake at 325°F for 8 to 10 minutes, watching for edges that are just barely golden rather than deeply browned.
Let the cookies rest on the baking sheet for a minute before moving them to a wire rack. They need to be fully cool to the touch before any icing goes on, or your colors will run and blur together.