Preheat the oven to 160°C / gas mark 3. Line a large baking sheet with parchment paper. In a large bowl, whisk together the plain flour, cornmeal, baking powder, and salt.
In another large bowl, beat the caster sugar and eggs with an electric mixture until pale yellow, about 3 mins. Mix in the lemon zest and then the plain flour, and beat until just blended (the dough will be sticky). Stir in the almonds. Let the dough rest for 5 mins.
Divide the dough evenly into 2 equal mounds and place on the prepared baking sheet. With moist hands, space the dough evenly apart and form into 2 (22 by 7.5cm) logs. Bake for 35 mins until lightly browned. Cool for 5 mins.
Using a serrated knife, cut the logs crosswise into 10cm thick diagonal slices. Arrange the biscotti cut side down on the same baking sheet. Bake until the cookies are pale golden, about 25 mins. Let cool completely.
Place the chocolate chips in a medium bowl. Place the bowl over a pan of simmering water, making sure the bottom of the pan does not touch the water. Stir until the chocolate is melted and smooth. Dip the end of each biscotti in the chocolate. Transfer the dipped biscotti to a wire rack, set over a baking sheet, until the chocolate has hardened. Store in an airtight container.