Cream butter and sugar on med-high until fluffy. Add egg and beat until mixed, then reduce speed to low and add the sour cream, vanilla, and almond extract. Make sure to scrape down the sides if necessary.
In another bowl, mix flour, cornstarch, baking soda, baking powder, cream of tartar, and salt. slowly add dry ingredients to wet ingredients until fully combined. Cover the bowl and refrigerate for at least 1 hour.
Preheat oven to 375°F, and line 2 baking sheets with parchment paper. Roll tbsp-sized scoops of dough into a ball, and then place on the baking sheet, spaced out. Use the bottom of a glass to flatten the dough into a disk (the dough is sticky, use parchment between the dough and the glass).
Bake for 9-13 min until just starting to turn golden. Allow to cool completely on a wire rack before frosting.