In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or a large bowl, beat together the butter and sugar until pale and fluffy.
Beat in the egg, scraping down the sides of the bowl as needed, then the vanilla extract, almond extract, and sour cream.
Add the flour mixture and mix just until the dough comes together with no streaks of flour remaining.
Form the dough into a disc, wrap in plastic, and refrigerate for one hour, until chilled.
Preheat oven to 350˚F (180˚C). Line two baking sheets with parchment or lightly grease.
Place a piece of parchment on a work surface and dust with flour.
Add the chilled dough, dust the top with flour, and roll into a sheet about 1/4 inch (6 millimeters) thick.
Cut the sheet into individual cookies using a round (or oval for Easter Eggs) cutter about 3 inches (7.6 centimeters) long. Dip the cutter into flour as needed to prevent sticking.
Arrange the cut cookies on the baking sheets about 2 inches (5 centimeters) apart.
Bring together the scraps of dough, roll out again, and cut out more cookies.
Bake the cookies in the preheated oven just until set and lightly golden on the bottom, 8-10 minutes.
Remove from the oven and cool on the baking sheets for 5 minutes before removing to a wire rack to cool completely.