Preheat oven to 350.  Line baking sheet with parchment paper.

For the Cookie:  Cream butter with brown sugar and white sugar until light and fluffy.  Add in 2 eggs and mix to combine.  Add the dry ingredients of flour, cornstarch, cream of tartar, baking powder, baking soda, salt, and cinnamon.  Mix until all ingredients are well incorporated.

Scoop balls of dough, about 1 Tablespoon in size, and roll in a mixture of 1/4 cup sugar and 1/2 teaspoon cinnamon.  Place on baking sheet about 2-3 inches apart, leaving room for cookies to spread.  Bake for 8-10 minutes and remove from oven when edges have started to turn golden brown, but the cookie is still a bit soft.  Allow cookies to cool on baking sheet for 5 minutes before transferring to cooling rack to cool completely.

For the Buttercream:  Once cookies have cooled fully, mix 1 cup butter, 2 cups powdered sugar, pinch of salt, 1 teaspoon vanilla, and 1 teaspoon cinnamon until light and creamy.  Pipe or spread buttercream over the cookies.

Sprinkle cookies with additional cinnamon and sugar before serving.