Preheat and prep: Set the oven to 350°F (175°C). Line two baking sheets with parchment paper.

Cream the butter and sugar: In a large bowl, beat the softened butter and 1 1/4 cups sugar until light and fluffy, about 2–3 minutes. This traps air and helps the cookies rise and stay tender.

Add eggs and vanilla: Beat in the egg, egg yolk, and vanilla until smooth. Scrape down the bowl to make sure everything is evenly mixed.

Whisk dry ingredients: In a separate bowl, whisk the flour, cream of tartar, baking soda, and salt until combined.

Combine wet and dry: Add the dry ingredients to the wet in two additions. Mix on low until just incorporated. The dough should be soft and slightly sticky but manageable.

Mix the coating: In a small bowl, stir together 1/4 cup sugar and 2 teaspoons cinnamon.

Shape and roll: Scoop the dough into 1 1/2-tablespoon portions (a medium cookie scoop works well). Roll each into a ball, then coat generously in the cinnamon-sugar mixture.

Optional chill: For thicker cookies, place the coated dough balls on a tray and chill for 20–30 minutes. If you want thinner, crinkly cookies, skip the chill.

Bake: Arrange the dough balls on the prepared sheets, leaving about 2 inches between each. Bake for 9–11 minutes, until the edges are set and the centers look slightly underbaked and puffy.

Cool: Let the cookies rest on the baking sheet for 5 minutes, then move them to a wire rack to cool. They’ll firm up as they cool but stay soft inside.