Preheat oven to 375°F. Line baking sheets with parchment paper (this helps the cookies from spreading).
Using a hand mixer, or stand mixer fitted with the paddle attachment, beat butter and sugar for 2 minutes, until light and creamy. Add extracts, egg, egg yolk, and sour cream and beat until combined, scraping the sides of the bowl as necessary. The mixture might look a little curdled, that’s ok.
In a separate bowl, combine flour, cornstarch, baking powder, baking soda, and salt. Slowly add the dry ingredients to the wet ingredients and beat until combined.
Allow the dough to sit at room temperature for 5 minutes. Scoop the dough into 2-tablespoon balls (briefly rolling them in your hands, if necessary, to make the dough balls smooth). Place the dough balls a couple inches apart on the prepared baking sheets.
Bake for 9-11 minutes (see note), until the edges are set and the center is still slightly underdone. The cookies should still be light in color, not brown. Place the baking sheets on wire racks to cool completely; the cookies will firm up once cool.