Preheat oven to 180 degrees C. Grease a baking tray.
In a medium bowl stir together flour, sugar, baking powder, bicarbonate of soda and salt. Beat in the egg and egg white then mix in almonds and orange zest.
Knead dough by hand until mixture forms a smooth ball.
Roll the dough into a log about 25cm long then place on the prepared baking tray. Press down or roll with a rolling pin until log is 15cm wide.
Bake for 25 minutes in preheated oven. After baking cool on a rack. With a serrated knife cut into 2cm slices. Place slices cut side down back onto the baking tray.
Return them to the oven for an additional 20 to 25 minutes turning over half way through the baking.
Melt the chocolate over a double boiler or in the microwave oven. Allow chocolate to cool but not harden before dipping one side of the biscotti into it. Place biscuits on wire racks chocolate side up until cool and dry.