Beat the butter and sugar until creamy. Add the flour, baking powder, coriander, orange rind and juice, polenta and egg and mix to form a firm dough. Knead in the almonds. Turn out the dough on a lightly floured surface and gently work in the chocolate chips. Divide the mixture in half and shape each piece into a log about 23 cm (9 inches) long.
Place on a greased baking sheet, spaced well apart, and flatten slightly. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 30 minutes or until risen and just firm. Remove from the oven and cool on the baking sheet for 15 minutes. Transfer to a board and, using a serrated knife, cut into 1 cm (½ inch) thick slices.
Arrange on the baking sheet, cut sides down, and bake for 15 minutes until crisp. Transfer to a wire rack and leave to cool.
Melt the plain dark and white chocolate in 2 separate heatproof bowls over pans of simmering water. Line a tray or clean baking sheet with nonstick baking paper. Dip about one-third of each biscuit in the plain dark chocolate, letting the excess fall back into the bowl. Place each biscuit on the baking paper.
Drizzle thin lines of melted white chocolate over the plain dark chocolate to decorate. Leave in a cool place to set for about 1 hour.