Prepare the Baking Sheets: Preheat the oven to 350°F (175°C) and line the baking sheets with parchment paper; set them aside.
Cream the Butter and Sugar: In a large mixing bowl, use an electric mixer to beat the softened butter and granulated sugar until thoroughly combined.
Add the Eggs and Vanilla: Add the egg, egg yolk, and vanilla extract to the butter mixture, then stir until everything is well incorporated.
Combine the Dry Ingredients: In a separate medium-sized bowl, whisk together the flour, cream of tartar, baking soda, and table salt until evenly combined.
Make the Cookie Dough: Gradually add the dry mixture to the wet mixture, stirring until the ingredients are completely combined and a uniform cookie dough forms.
Prepare the Cinnamon Ripple: In a separate medium-sized bowl, whisk the brown sugar, cinnamon, and salt together. Add the very soft or barely melted butter and mix until the mixture forms a paste. Create the Ripple: Dollop several spoonfuls of the cinnamon paste over the cookie dough, then gently fold it in with a spatula. Avoid overmixing so distinct cinnamon ribbons remain visible throughout the dough. Portion the Dough: Use a cookie scoop to portion the dough into level 1 ½-tablespoon-sized scoops. If desired, gently roll each portion between your palms to form a smooth ball. Arrange the Cookies: Place the cookie dough balls on the prepared baking sheets, leaving at least 2 inches of space between each one.
Bake the Cookies: Transfer the baking sheets to the center rack of the preheated 350°F (175°C) oven and bake for 10–11 minutes.
Cool Completely: Leave the baked cookies on the baking sheet until they have cooled completely before serving.
