For the espresso cookies, cream butter and sugar with the whisks of a hand mixer. Stir in eggs one at a time, mix in espresso powder, vanilla, lemon zest, salt, baking soda, flour and 100 g hazelnuts. Stir in briefly.
Divide the dough in half. Stir cocoa into one half, cut out one tablespoon each of light and dark dough. Place on a baking tray lined with baking paper to form a dough heap. Mix the doughs a little at a time. Sprinkle remaining nuts on top and flatten dough mounds slightly.
Bake in a convection oven at 160°C for about 15 minutes until the espresso cookies are lightly browned. Allow to cool.