Preheat your oven to 190°C (375°F/Gas 5). Cream the butter with the icing sugar and vanilla until it is smooth. Add the flour in four portions, mashing it in well with a wooden spoon after each addition. Mix the nuts through.
Divide the dough into quarters. Dip your fingers in icing sugar and then, from each quarter of dough, roll 10 balls the size of cherry tomatoes. Put on two ungreased baking trays, leaving some space between each ball. Flatten each one a bit, and then dip your thumb in icing sugar and make an indent in each ball. The icing sugar will sit in here and, even if you’re not icing the biscuits, it’ll look good.
Bake one tray at a time for about 15 minutes or until the biscuits are firm and golden, and slightly darker around the edges. Bake for a few more minutes, if necessary. They are delicate, so carefully lift them off the trays and onto racks to cool. Wait until they are completely cold if you are going to ice them, otherwise store them in a biscuit tin.
To make the icing, cream together the icing sugar, butter, vanilla and milk until completely smooth. You may need to use more or less milk to get a soft, buttery consistency. Add the food colouring, mixing it in well. If you are not ready to ice the biscuits immediately, cover the bowl with a wet paper towel.
Use a small butter knife or a teaspoon to spread the icing quite rustically into and over the indentations in the biscuits. When the icing has hardened a little, store the biscuits in a tin. They keep very well at room temperature.