Using electric beaters, beat the butter and sugar in a large bowl until the mixture is light and fluffy.
Sift the flour, cocoa powder, cinnamon and baking powder; add the ground walnuts. Add the dry ingredients to the butter mixture. Knead with floured hands to form a smooth dough.
Shape the dough into 8 cylinders, each about 40 cm long. Wrap cylinders in cling wrap; chill overnight in the refrigerator.
Preheat oven to 180°C. Line 2 large baking trays with baking paper.
On a floured work surface, cut each dough cylinder into 10 even pieces and shape into evenly sized balls. Place on baking trays. Make an indentation in the top of each ball for the walnuts.
Beat the egg white in a bowl; dip bottoms of walnut halves in it. Press the walnut halves into indentations. Bake the biscuits a tray at a time for 12–15 minutes until golden brown. Remove from oven, cool slightly; transfer to a wire rack to cool completely.