In a large bowl using either a hand mixer or a stand mixer with the paddle attachment, cream the softened butter on medium speed for a minute or so, until creamy. Then add the 3/4 cup sugar and beat until fluffy and light in color. Beat in the egg and vanilla extract. Scrape down the sides of the bowl as needed. Set this aside.
In a separate bowl, whisk together the flour, salt, baking powder, baking soda and cream of tartar. With your mixer running on low speed, add the dry ingredients to the wet in 3 parts until it is all incorporated.
Roll the dough into balls, about 1 tbsp. of dough per ball. In a small bowl, whisk together the 1/4 cup sugar and the cinnamon. Roll each dough ball into the cinnamon sugar mixture to coat. Then press in 5 almonds slivers per dough ball, as pictured, in a sand dollar design.
Place the dough balls in the fridge to chill for at least 2 hours and up to 48 hours. Once you're ready to bake, preheat the oven to 350F. Line a couple large baking trays with either parchment paper or silicone baking mats. When placing the dough balls on the prepared trays, make sure to leave plenty of room between them, about 3 inches. I baked 6 at a time on a half sheet size tray.
Bake for 8-10 minutes. The cookies will appear still very soft and undone. Allow the cookies to cool on the baking trays for 3 minutes, then transfer them to a wire rack to cool completely.
Store these cookies for up to 1 week in a covered container at room temperature.
