Put the flour, sugar and hazelnuts in the food processor with a pinch of salt. Add the butter and process to fine breadcrumbs. With the processor running, add the egg and whizz until the dough forms a ball. You may need to add a splash of cold water if it doesn’t come together quickly. Roll the dough out thinly on a floured work surface. Use a pastry cutter to cut 24 small circles about 8cm across. Press each into a mini muffin tin and put in the freezer for 20 minutes. Heat the oven to 180c/160c fan/gas 4. Bake the tart cases for 15 minutes.