Prepare the caramel filling by shaping small squares of caramel. Place them in the freezer for easier stuffing later.
In a skillet, melt butter over medium heat until golden brown and nutty. Transfer to a bowl and let cool slightly.
Whisk flour, cream of tartar, baking soda, baking powder, cornstarch, salt, and cinnamon in a separate bowl.
Add brown sugar and granulated sugar to the butter, whisking until smooth. Beat in egg, yolk, vanilla, and sour cream.
Fold in dry ingredients until just combined. Scoop dough into 16 equal portions.
Flatten each ball, place a caramel in the center, and seal tightly. Roll into a smooth ball.
Combine sugar and cinnamon for the topping, then roll each dough ball until coated.
Chill for 2 hours for thicker cookies, or bake immediately.
Bake at 350°F for 9–11 minutes, until edges are set but centers look soft.
Cool on baking sheet for 15 minutes before serving.