Preheat the oven to l80ºC. Cream the butter and sugar together until light and fluffy. Add the eggs, one at a time, followed by the chocolate powder.
Combine the flour, baking powder and salt; add to the chocolate mixture with the lemon zest, aniseeds, chopped almonds and Grand Mamier, if using. Mix well.
Divide the dough in half. With lightly moistened hands, shape each half into a 22x5cm log. Flatten the logs slightly.Wrap the dough with cling film and refrigerate for 1 hour.
Cut the logs into long, thin slices and place on a silicone mat- or paper-lined baking tray Bake for 10 minutes - or longer for crisper biscotti. Remove to wire racks to cool completely before storing in an airtight container.