In a large bowl, stir together flour, almond flour, sugar, cornstarch, baking soda, and salt
Add the cubed cold butter to the flour mixture and cut in the butter with a pastry cutter or fork. When the largest piece of butter is about the size of a pea it's good.
Mix in the 2 beaten eggs along with the almond extract until fully incorporated. The dough will be quite sticky.
Flatten the cookie dough into a disc on a sheet of plastic wrap and then wrap it all up. Then chill in the fridge for at least 2 hours or overnight
Once the cookie dough has been chilled, preheat your oven to 325 degrees F and line a baking sheet with parchment paper. (I like to use 2 trays - see note) In a small bowl, beat together the egg and water to make an egg wash.
Divide the cookie dough into 36 even pieces and then roll them into balls. Place the dough balls on the prepared baking sheet about 2-inches apart. (They will spread.) Lightly press an almond slice into the top of each cookie and then brush the cookies with egg wash.
Bake for 15-18 minutes, or until the cookies are just starting to turn golden brown.
Allow the cookies to cool for at least 5 minutes on the baking sheet before transferring them to a wire rack to cool completely. Store in an airtight container for up to 1 week.
Notes
The chill time is necessary for these cookies. The colder they turn out when baked. I stored the cookie dough in the fridge between batches. Also, the dough is much easier to work with when cold - AKA less sticky.
Make sure the baking sheet is cool before adding another round of cookies to it. If the pan is already warm the cookies spread much thinner. It's a good idea to rotate 2 baking sheets so one has time to cool while the other is in the oven.
You can use whole almonds instead of sliced almonds on top for more crunch.