Preheat oven to 350 degrees F. Line a baking sheet with parchment paper or a silicon baking mat and set aside.
Whisk together the flour, baking soda, cream of tartar and salt in a large bowl and set aside.
In a separate bowl, combine 1 cup powdered sugar, 1 cup granulated sugar, 1 cup butter and 1 cup butter flavored shortening and beat until light and fluffy. Beat in the eggs and vanilla extract until fully combined, scraping down sides of bowl as needed.
Gradually add in the dry ingredients just until incorporated. Roll the cookie dough into 1 1/2 to 2 tbsp size balls and roll in granulated sugar to coat. Use a flat bottomed glass, dipped in granulated sugar to press down each cookie ball to about 1/2 inch thickness. Make sure cookies are about 2 inches apart when baking.
Bake for 10 to 12 minutes, just until the center of the cookies are set. The cookies will not brown, so do not over cook them! Let cool in pan for 5 minutes before transferring to a wire rack to cool completely.
For the Frosting:
Beat the butter in a large bowl until light and fluffy. Add in the powdered sugar, salt and extracts and mix until combined, about 3 minutes.
Gradually add in the heavy cream, beating until your desired consistency is reached.
Frost the cooled cookies and top with sprinkles if desired. Store in an air tight container at room temperature or longer in the fridge or freezer.
These cookies can be made ahead of time and frozen for up to 3 months.