Sift the flour and bicarbonate of soda together

In a separate bowl, beat the butter and sugars until soft and creamy

Combine the egg, flour mixture, peanut butter and salt

Add the butter and sugar mixture, and mix until smooth

Wrap the dough in foil or parchment and refrigerate for at least two hours, preferably overnight

Pre-heat the oven to 160ºC (325ºF / Gas mark 3)

Shape the dough into 3-cm (1 1/4-in) balls and place them 5cm (2 in) apart on baking sheets. Flatten slightly with a fork.

Bake for 15 minutes, or until golden

Remove from the oven and allow to cool

Store in an airtight container for up to five days