Spread the pistachios and pine nuts out on a baking sheet and toast in a preheated oven, 160°C (325°F), Gas Mark 3, for 5–10 minutes until golden. Remove the nuts and leave to cool but leave the oven on.
Beat the butter and sugar together in a large bowl until just mixed, then beat in the eggs, lemon rind and Amaretto. Sift the flour, baking powder and salt together into a separate bowl, then stir into the butter mixture with the polenta. Stir in the toasted pistachios and pine nuts.
Turn the dough out on to a floured work surface and knead until smooth, working in a little more flour if too sticky. Divide into quarters and roll each quarter into a sausage 5 cm (2 inches) long and 1.5 cm (¾ inch) thick. Flatten slightly. Place on 2 greased baking sheets and bake in the oven for about 35 minutes until just golden around the edges.
Leave to cool slightly, then cut on the diagonal into 1 cm (½ inch) thick slices. Place, cut-side down, on the baking sheets and bake for a further 10–15 minutes until golden brown and crisp, being careful not to burn. Transfer to a wire rack to cool.