Whisk together the flour, cocoa powder, baking powder and salt.
Beat the butter and sugar in a medium bowl with until just combined.
Mix in the yolk.
Add the dry ingredients to the butter and beat lightly together until just combined but still crumbly.
Fold in grated chocolate with a spatula.
Bring the dough together by lightly squeezing in your hands; but don't knead or overwork.
Divide the dough in half.
Lay half the dough on a long sheet of waxed paper, roll up and shape into a log.
Repeat with remaining dough and refrigerate for at least 1 hour.
Preheat the oven to 180C.
Line 2 baking sheets with bakingt paper.
Slice logs into 1/2-inch thick rounds with a sharp, thin knife. Divide rounds onto the prepared sheets, leaving about 1 inch between them, and refrigerate for 15 minutes.
Bake about 12 to 14 minutes. Remove from the oven and let cool on the pans, about 5 minutes.
