Process almonds and sugar in small batches until finely grind. Make sure to process a little sugar and little almonds at a time. Ideally, for medium size food processor (2 quarts or 2 liters cup) not more than ½ cup almonds and ¼ cup sugar at one go. Pay attention to not overly process or you'll get a sort almond butter. Stop frequently (4-5 times) scraping the sides of the bowl.
Place finely grind almond and sugar mix in a bowl.
In a separate bowl beat egg whites at room temperature until stiff peaks - the foam should hold its shape well.
In a bowl with ground almonds and sugar mix add a vial of bitter almond extract. Make sure you use those sold in vials and not the extract sold in bottles.
Start adding whipped egg whites, a few dollops at a time. Mix until absorbed then add more. As a final result the dough should soft but hold its shape.
Let the dough rest at room temperature for 5 hours or overnight covered with a plastic wrap. As the dough rests it will thicken slightly without becoming hard. It's the key step for obtaining beautiful cracks and a perfect cookie shape that doesn't flatten out.
Roll the balls about 35 grams each (about 1.2 ounces). Roll each ball in granulated sugar and arrange on a baking sheet lined with parchment paper. Decorating with a whole almond slightly pressing it into the dough.
Bake in a preheated for 320 F (160C) oven on static setting (fan off) for 15-20 minutes. Cookies should be slightly browned on top and on the bottom.
Let cool completely and enjoy.
Notes
If you want to use bitter apricot kernels instead of bitter almond extract use 45 grams of apricot kernels and 255 grams of blanched almonds. Mix them together and grind with sugar as directed in the recipe instructions.For the perfect result always use a kitchen scale to measure all ingredients in grams.