Preheat oven to 180°C/350°F/Gas Mark 4. Line two baking trays with baking paper. Blend butter, sugar, egg and chilli in a food processor until creamy. Add flours, baking soda and salt and mix to form a dough. Tip the dough into a bowl and stir in chocolate, nuts and half the ginger. Divide the mixture into 30 even-sized pieces and place 15 pieces of dough on each tray, leaving room for spreading. Sprinkle remaining ginger over biscuits, then bake until golden brown, about 20 minutes. As you take the trays from the oven, bang them on a bench to release air. Cool biscuits on the trays. Store in an airtight tin for up to 2 weeks.