Preheat oven to 180 C / Gas 4. Mix together the flour, baking powder, bicarbonate of soda and salt in a bowl; set aside. Grease two baking trays.
Beat the butter and sugar with an electric mixer until fluffy. Beat in one egg until completely incorporated, then beat in the other. Beat in the last egg along with the anise extract. Mix in the flour until just incorporated. Divide the dough onto the prepared baking trays. Form each portion into a 30cm long log, 1.25cm thick.
Bake in the preheated oven until golden and firm to the touch, 20 to 25 minutes. Remove from the oven and allow to cool 5 minutes. Cut into 1.75cm thick slices using a serrated knife and place cut-side-down onto the baking trays. Return to the oven and bake 5 to 10 minutes until the bottoms turn golden brown. Turn the biscotti over and continue baking until golden brown on the other side, 5 to 10 minutes more. Cool completely on a wire rack before serving.