Mix the flour and semolina in a bowl. Rub in the butter with your fingertips until the mix resembles breadcrumbs. Stir in the sugar and 5 tbsp cold water and bring together with your hands. Wrap in cling film. Chill for 30 minutes.
Preheat the oven to 180°C, gas mark 4. Bring 500ml water to the boil, add the dates and simmer for 3 minutes. Drain, reserving 5 tbsp liquid. Dissolve the coffee in this. Blend the dates and coffee with a hand blender, then add the walnuts.
On a floured surface, roll out the dough to 5mm thick. Using an 8cm round cutter, cut out 24 rounds. Transfer to the tin – the dough will hang over the edge. Fill each with 1 tsp date mix. Brush the edges with water and seal.
Bake for 25-30 minutes, until golden. Leave to cool, then dust with icing sugar.