Heat oven to 180C/160C fan/gas 4 and line 2 large baking trays with baking parchment. Measure the peanut butter and sugar into a bowl. Add ¼ tsp fine table salt and mix well with a wooden spoon. Add the egg and mix again until the mixture forms a dough.
Break off cherry tomato sized chunks of dough and place, well spaced apart, on the trays. Press the cookies down with the back of a fork to squash them a little. The cookies can now be frozen for 2 months, cook from frozen adding an extra min or 2 to the cooking time. Bake for 12 mins, until golden around the edges and paler in the centre. Cool on the trays for 10 mins, then transfer to a wire rack and cool completely. Store in a cookie jar for up to 3 days.