[Make cookie batter.]
Cream unsalted butter until smooth, add granulated sugar in three batches, and mix well each time.

Use an electric hand mixer to mix on low speed for 4 minutes until whitish. Mix for 2 mins, scrape down the sides of the bowl and mix for 2 mins.
Add beaten egg and mix on low speed until smooth.

Add ground almonds and use a spatula to mix until smooth while crushing lumps.
Divide into four equals, each about 80-85g (2.8-3 oz).

Sift 75g of flour and 10g of vegetable powder into one butter mixture. Fold it until it turns an even color while scooping the bottom flour and cutting the batter. It should look like dry bread crumbs.

Gather the dough in the center, and use a spatula to press down. Repeat it (gather, press) two more times.
Put it out on the work surface lined with plastic wrap. Use your hands to press it until it comes together, and flatten it.

Fold it in half and press. Repeat ''folding in half and pressing'' 7-10 times until smooth.

Use a rolling pin to roll it out to 1 cm / 0.4'' thick, 11-12 cm / 4.3 - 4.7 inches square.
In the same way, use the remaining flour and each vegetable powder to make 3 square doughs. Let it sit in a fridge for 1 hour and a half.

[Cut the dough.]
Take it out of the fridge. If the room is hot, take out the dough one by one and work on it.
Dust a knife with bread flour, shaking off the excess flour. Cut off the 4 sides with a width of 5 mm / 0.2''. Cut into 9 equal squares. Dust bread flour on the knife from time to time.

Arrange them on a baking sheet lined with a baking mat (parchment paper) and let them sit in a freezer for 1 hour.
Let the remaining cut cookies which couldn't be put on the baking sheet, let it in a freezer, and bake in next.

[Bake]
Preheat an oven to 190℃ / 374 F. Gently brush off the excess flour from the surface just before baking.
Lower the oven to 170℃ / 338 F and bake for 20 minutes until the edges turn lightly brown.

Remove from the oven and let cool for five minutes, then transfer to a rack to cool completely. After cooling, store them in an airtight container at room temperature.