Preheat oven to 350°F and line a baking sheet with parchment paper.
In a medium bowl, stir together the 3 tablespoons (42g) unsalted butter and 3 tablespoons (60g) granulated sugar until smooth and combined. If the butter is softened (not melted), use the back of the spoon to press and mash everything together until creamy.
Mix in the 1 large egg yolk and 1 teaspoon vanilla extract until well combined.
Add the 2/3 cup (80g) all-purpose flour, 1 teaspoon cream of tartar, 1/4 teaspoon baking soda, and pinch of salt. Stir until a soft dough forms. It may look a little flour-heavy at first, but keep mixing and it will come together.
In a small bowl, mix the 1/2 tablespoon ground cinnamon and 2 tablespoons granulated sugar for rolling.
Scoop dough into 6 equal portions (34 grams each), roll into balls, then coat in cinnamon sugar, and place on the baking sheet.
Bake for 10-12 minutes, until edges are set and centers look slightly soft. The cookies will look puffy when they come out of the oven, but as they cool, they’ll flatten a bit and develop soft, crackly tops.
Let cool on the pan for a few minutes before transferring to a cooling rack.