Makes 12-15 biscuits
Preheat the oven to 180C / 356 F / Gas Mark 4 and line a large baking sheet with baking paper.
Cream together the butter and both sugars until light and fluffy. Add in the egg yolk and vanilla and combine well. Then mix in the flour and salt. Pop the dough in the fridge for about 20 minutes.
Meanwhile in a small dish, whisk the egg white with 1/2 teaspoon of water. Put the pistachios on a flat plate. Take the dough out of the fridge and pinch off a tablespoon sized piece of dough. Roll it between your palms to create a ball. Brush it with the egg wash and roll it in the chopped pistachios. The egg wash will prevent the nuts from falling off the dough.
Place each dough ball onto the baking sheet and using your thumb make an indent. Fill the indent with 1 teaspoon of jam making sure the well is nicely filled as the biscuits will flatten out as they bake. Repeat until all the dough is used up, setting the biscuits approximately 2.5 cm (1 inch) apart on the baking sheet.
Place in the oven for 12-14 minutes. The biscuits will set once you remove them from the oven. After 5 minutes put them on a wire rack to cool.
Next make the icing. Mix together the icing sugar and milk, adding more if the icing is too thick. Drizzle the icing over the cookies and allow to set before serving.