Finely grind 1 cup blanched almonds using a rotary hand grater.
Put the ground almonds in a large bowl and add 1 egg white and 1 ⅔ cups powdered sugar. First, mix with a fork, then work by hand until you have a soft and slightly sticky cookie dough.
Divide the dough into two halves. Work 1 ½ Tablespoons cocoa powder into one half, which may require some patience. Your hands will get covered with cocoa powder, but you can easily wash them under warm water afterward.
Roll both doughs into long, equally thick cylinders. Place them side by side, leaving about 1 inch (2.5 cm) of space between them.
Using a dough scraper or knife, cut the cylinders into equal-sized pieces, slicing both halves at the same time to ensure you get the same number of white and cocoa bits.
Mushroom caps: Roll the cocoa piece into a ball. Using the end of a wooden spoon with a smaller handle diameter, carefully make an indent in the center of the ball.
Mushroom stems: Roll the white piece into a ball and gently shape it into a cylinder. Pull the top part into a point and slightly flatten the bottom to make the mushroom stand upright.
Lastly, insert the stem, pointy side up, into the hole in the mushroom cap and press gently to secure it.