Melt the chocolate in a bowl over a pan of simmering water (see
Sift the flour and baking powder into a bowl. Add the sugar, semolina or polenta, orange rind, egg, vanilla essence and walnuts. Add the melted chocolate and butter and mix to form a dough. If the mixture feels dry, add 1 tablespoon water.
Turn the dough out on to a lightly floured surface and divide it in half. Shape each half into a sausage, about 28 cm (11 inches) long. Lightly grease a large baking sheet. Transfer each sausage to the baking sheet and flatten to about 1.5 cm (¾ inch) thick.
Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for 25 minutes or until risen and firm. Leave to cool, then diagonally slice each slab into 1.5 cm (¾ inch) thick biscuits. Return the biscuits to the baking sheet, spacing them slightly apart, and bake for a further 10 minutes until crisp. Leave to cool, then dust with icing sugar.