1 cup (2 sticks) unsalted butter
2 1/2 cups all-purpose flour
1 teaspoon baking soda
2 teaspoons cream of tartar
1/2 teaspoon cinnamon
1/4 teaspoon salt
1 3/4 cup dark brown sugar
1/2 cup granulated sugar
1 large egg
1 egg yolk
1 tablespoon vanilla extract
1 tablespoon plain Greek yogurt (I used regular vanilla yogurt)
For rolling cookies in: 1/2 cup granulated sugar + 3 tablespoons cinnamon
Place butter in a saucepan over medium heat and whisk constantly until butter foams up and turns golden brown (more info on browning butter here). Immediately pour into a small bowl and allow to cool for several minutes.
In a medium bowl, whisk together the flour, baking soda, cream of tartar, cinnamon, and salt. Set aside.
With an electric mixture, beat the cooled brown butter with the brown sugar and granulated sugar until thoroughly blended. Beat in the egg, egg yolk, vanilla, and yogurt until smooth. Add the dry ingredients slowly until just combined.
Chill dough for 30 minutes in the refrigerator.
Preheat oven to 350°F.
Spray a cookie sheet with nonstick baking spray. In a small bowl, combine 1/2 cup sugar with 3 tablespoons cinnamon.
Roll dough into 1 1/2-tablespoon balls. Roll in cinnamon-sugar, place on baking sheet 2-inches apart, and flatten slightly.
Bake for 8-11 minutes or until cookies begin to turn golden brown. They will look a bit underdone in the middle, but will continue to cook once out of the oven. Bake longer if you like crispier cookies. Cool cookies for two minutes, then remove to wire racks to cool completely.