Preheat oven to 350°F (177°C). Spray cookie sheet with nonstick spray or use a silicone baking mat or parchment paper.
In a medium-sized mixing bowl, stir the butter and 3 Tablespoons sugar together until creamed—mixture may be a bit crumbly. That’s OK. Add the egg and vanilla until mixed. Stir in flour, baking soda, salt,cream of tartar, and 1/4 tsp cinnamon until just combined. Do NOT overmix.
Place the dough in the center of the prepared cookie sheet into a tall ball (see pictures above). Sprinkle with 1/4 teaspoon cinnamon and 1/4 teaspoon sugar. Bake for 14-16 minutes until the edges are browned. The center may appear undone but that’s what you want. It will firm up. Allow to cool completely ON the baking sheet. Enjoy!