Preheat oven to 170°C/340°F/Gas Mark 3–4. Line two baking tray with baking paper. Cream butter with lemon zest and icing sugar. Sift in flours, polenta, paprika and baking powder. Add fennel and sesame seeds and mix to combine. Roll walnut-sized pieces into balls. Roll each ball in brown sugar to lightly coat then place 4 cm (1 ½ in) apart on the baking tray and press down lightly with your fingers. Bake 15–20 minutes until golden. Cool on the tray. Store in an airtight tin for up to 2 weeks.