½cupalmond slivers(you won't need all of these slivers, but it's better to have extra as not all of the slivers will be ideal for sand dollar stars and it's better to have more to choose from)
In a large bowl, use an electric mixer (or stand mixer) to cream together butter and sugars.
Add egg yolk only (save the egg white, you’ll need it later!) and vanilla extract and stir until well combined.
In a separate bowl, whisk together flour, salt, cream of tartar, baking soda, and ground cinnamon.
With mixer on low-speed, gradually add flour mixture to butter mixture until completely combined. This is a fairly dry dough, make sure to scrape the sides and bottom of the bowl as needed and stir until no flour remains.
Divide dough into two parts and form each into a ball. Flatten into a disk and wrap in plastic wrap. Chill in the refrigerator for at least 15 minutes and up to 3 days.
While dough is chilling, preheat oven to 375F (190C) and prepare cinnamon/sugar mixture by whisking together ¼ cup (50g) granulated sugar and 1 ½ teaspoons ground cinnamon.
Working with one half at a time, remove chilled dough from the fridge and transfer to a clean, lightly floured surface. Use a rolling pin to roll dough to be slightly thinner than ¼” (0.6cm). (Note: As you roll out the dough, periodically use a spatula to lift the dough from the counter and re-flour underneath as needed to keep the dough from sticking to the counter. If your dough feels too sticky to use, simply return it to the fridge and allow it to chill longer).
Use a 2.5” (6cm) round cookie cutter to cut circle-shaped cookies and transfer to a parchment paper lined baking sheet, spacing cookies at least 1” apart.
Use a pastry brush to lightly brush the surface of each cookie with your reserved egg white. Sprinkle cookies lightly with cinnamon/sugar mixture (you may not use all of this mixture) and then take your almond slivers and gently but firmly press 5 slivers in a star-shaped pattern into the center of each cookie.
Transfer cookies to 375F (190C) preheated oven and bake 8-10 minutes or until cookies are beginning to turn a light golden brown around the edges.
Allow to cool completely on baking sheet before enjoying.
Store in an airtight container at room temperature for up to 10 days.