Preheat oven to 375°F with a rack in the upper and lower third of the oven. Line two baking sheets with parchment paper.
Using a hand mixer, or stand mixer fitted with the paddle attachment, beat butter and sugar for 2 minutes, until light and creamy. Note: the butter should be just soft enough to beat smoothly but still cool. 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, to make the dough balls smooth). Place the dough balls a couple inches apart on the prepared baking sheets.
Using the bottom of a wooden spoon or similar instrument, form an indention on top of each dough ball. This will help create the start of the cavity we will use to fill the cookies. The dough should still be in a ball shape, not flattened.
Bake for 10-12 minutes (see note about chill time), until the edges are set and the center is still slightly underdone. If necessary, rotate the baking sheets once during bake time. The cookies should be light in color and very delicate. Place the baking sheets on wire racks. As soon as they come out of the oven, use a slightly larger instrument to further form the cavity in the center of each cookie. I have successfully used the rounded bottom of a tablespoon as well as the flat bottom of a small espresso shot glass. The very center of the cookies (where you made the indentation) will look a little under baked, but they should be fine once they cool. Leave the cookies on the baking sheets to cool completely.