Preheat oven to 350 degrees F and prepare a 9x9 inch square cake pan with parchment paper
In a medium bowl combine the flour, salt, baking powder and baking soda; set aside
In a mixing bowl with a paddle attachment, cream together the unsalted butter and granulated sugar until smooth and fluffy
Add in the egg and egg yolk to the creamed butter and sugar and mix until fully combined
On low speed, slowly add in the dry ingredients to the butter mixture until fully combined
Just before the cookie batter is finished mixing, add the sprinkles into the batter and mix on low speed until sprinkles are mixed throughout the batter
Remove the bowl from the electric mixer and scrape the sugar cookie batter into the prepared square cake pan
Press the sugar cookie batter into the pan until it is spread evenly
Bake the Sugar Cookie Bars for 14-16 minutes or until the top of the cookie has a very light golden edge
Remove the Sugar Cookie Bars from the oven and allow them to cool completely before removing them from the pan
While you are waiting for the Sugar Cookie Bars to cool, prepare the Buttercream Frosting