Preheat oven to 350°F. Line two large baking sheets with parchment paper.
In a medium bowl, whisk together the flour, cornstarch, baking powder, salt, and cinnamon until fully combined. Set aside.
Use a medium bowl and a hand mixer on low speed to cream together the soft butter and the sugars until well combined. Turn the mixer up to medium and beat 1 more minute to mix everything up well. Beat in the egg and vanilla extract. Add in the sour cream, mixing just until combined.
Pour the dry ingredients (flour mixture) into the wet (butter mixture) and use a spatula to fold in until just combined. The dough will be wet and a bit sticky.
Divide dough into 16 equally-sized balls, about three tablespoons each (or about 50g per cookie). Use a cookie scoop for this or just a spoon or your hands. Roll each cookie in the cinnamon sugar mixture and place on a baking sheet at least 3" apart. Gently press down with a small flat-bottom glass or your fingers until each cookie is about 1/2" thick.
Bake cookies one tray at a time for 10-13 minutes or until edges are set and lightly golden brown. We want the cookies to stay soft (so slightly underbaked). The centers will remain quite soft, but not gooey. Cookies will be a bit puffed and then deflate as they cool.
Let cool for 10 minutes then move to a cooling rack to cool for another 20-30 minutes.