Walnut and coffee tarts

Name: Walnut and coffee tarts
Prep time: 20 mins
Cook time: 20 mins
Yield: Makes 6
Ingredients: 250g plain flour, plus extra for dusting; 125g cold butter, in small cubes; 1egg yolk; 60g icing sugar; 250g sugar; 150g double cream; 50g salted butter; 200g walnut halves, 6 left whole, the rest chopped; 150g white cooking chocolate; 75g double cream; 1tbsp instant coffee granules; Crème fraîche