Using your hand mixer or a stand mixer, beat the butter and sugar on medium-high speed until nice and fluffy. Add egg and beat until incorporated. Reduce speed to low and add sour cream and vanilla and beat, until well combined, scraping down the bowl with a silicone spatula.
In a separate bowl, sift in the flour, cornstarch, baking soda, baking powder, cream of tartar and salt and whisk together. Turn the mixer on low and slowly add the dry ingredients to the wet ingredients and mix until combined. Cover the bowl with plastic wrap and refrigerate for 1 hour.
Preheat oven to 375 degrees F.
To make the butter cream frosting: Cream butter in a stand mixer or using your hand mixer for 3 minutes until light and fluffy. Add salt, vanilla then slowly add 2 cups of powdered sugar and beat until smooth. Add heavy cream, 1 tablespoon at a time then add the remainder of powdered sugar until desired consistency and taste is reached (you may not need it all) and continue to beat until smooth.
*Sugar cookies get hard and dry out if you don’t seal them up properly so it's important to store them in an airtight container.
*Total time does not include chilling