If the potatoes are hot, mash them, then stir in the butter until it has melted and is distributed evenly throughout the potatoes. Leave to cool.

Sift the flour and salt together and stir into the mashed potato to form a soft dough.

If the potatoes are cold, mash them without adding the butter. Sift the flour and salt into a bowl. Rub in the butter with your fingertips until the mixture resembles fine breadcrumbs, then stir the flour and butter mixture into the mashed potatoes to form a soft dough.

Either roll out or press the dough into a 25 cm (10 inch) circle on a lightly greased baking sheet (cookie sheet) or pizza pan, pushing up the edge slightly all round to form a ridge before adding the topping. This base is tricky to lift before it is cooked, so you will find it easier to handle if you roll it out directly on to the baking sheet (cookie sheet).

If the base is not required for cooking immediately, you can cover it with clingfilm (plastic wrap) and chill it for up to 2 hours.