Carefully transfer the dough logs onto one of the baking trays and bake for 25 minutes, until hard on the outside but not all the way through – this will allow you to slice the biscotti without the log crumbling. Leave to cool in the tin.
With a serrated knife, cut the biscotti diagonally into 1.5cm-thick slices. Arrange the slices on the baking sheets and bake for 15 to 20 minutes, until firm. The biscuits may still seem a little soft but they will firm up as they cool.
Melt the remaining chocolate in a bowl over a pan of gently simmering water. Drizzle it over the biscotti and leave for about 20 minutes to set. These will keep in an airtight container for a few days.