In a mixing bowl, stir together the flour, the 2/3 cup granulated sugar, the cocoa powder, baking powder, and coffee crystals.
Add the butter, egg yolks, and vanilla; beat until the mixture is well combined into a dough.
Form dough into a ball. Divide into 6 portions. Roll out each piece to a 12-inch-long rope (about 3/4 inch thick).
Place ropes on ungreased cookie sheets. Flatten slightly. Brush with egg white and sprinkle with pearl sugar.
Bake in a 350 degree F oven 12 minutes or until edges are firm (ropes will spread). While warm, cut into 1-inch pieces. Cool on cookie sheets. Makes about 72.