Preheat oven to 350degrees. Line one 8x8" or 9x9" baking pan with aluminum foil, set aside.
In the bowl of a stand mixer, cream the butter, sugar and vanilla until light and fluffy, about 2-3 minutes. With mixing speed on low, add the egg, and egg yolk.
In a separate bowl, whisk together the flour, salt, baking powder, baking soda, cornstarch, cream of tartar and cinnamon. With mixing speed on low speed, gradually add the dry ingredients to the wet ingredients, mixing until no flour pockets remain. Be careful not to overmix.
Using a rubber spatula or the palm of your hand, press the dough into the prepared baking pan. Bake for approximately 22-26 minutes or until very lightly browned on top (be careful not to overbake). Remove from the oven and allow the bars to cool in the pan for 30-45 minutes before frosting.
While the bars are cooling, prepare the frosting by placing the butter in the bowl of a stand mixer, Beat until light and creamy, about 2-3 minutes. With mixing speed on low, add the powdered sugar, cream, vanilla extract, cinnamon and salt. Beat on high for 2-3 minutes until it becomes fluffy.
Using the foil as handles, remove the cooled bars from the pan. Frost the bars with a thick layer of frosting and garnish with additional cinnamon and coarse sugar.