Heat the oven to 180C/160C fan/gas 4. Butter and line a 28 x 18cm tin. In a food processor, pulse together the butter, both flours, 1/2 tsp salt and the 100g of sugar until the mixture resembles fine breadcrumbs. Pulse a few more times until it comes together into a rough ball. Alternatively, put all the ingredients in a bowl and rub together using your fingertips until the mixture resembles fine breadcrumbs. Bring together into a pliable dough and press down into the prepared tin. Using a sharp knife, mark the dough into 16 bars.

Sprinkle over the remaining sugar and bake for 25-30 mins until golden and set. Remove and prick the dough all over. Leave to cool in the tin for 5 mins before transferring to a wire rack to cool completely. Using a sharp knife, cut through your earlier scores to separate completely.