Makes 24 cookies
115 g (4oz) unsalted butter, softened
110 g (4oz) castor sugar
100 g (3 1/2 oz) soft light brown sugar
1 large egg
125 g (4 1/2 oz) peanut butter, smooth or crunchy depending on taste
165 g (6 oz) plain flour
3/4 teaspoon bicarbonate of soda
1/4 teaspoon salt
1/4 teaspoon baking powder
50 g (1 3/4 oz) mini marshmallows or chopped large marshmallows
Preheat the oven to 175°C (350° F), gas 4. Line three large baking sheet with nonstick baking parchment.
Using a freestanding electric mixer with a paddle attachment or a handheld electric whisk, cream the butter and sugars together for around five minutes on a medium to high speed until light and fluffy.
Add the eggs and peanut butter to the mixture and beat for a further two minutes.
Sift together the dry ingredients and add them to the butter and sugar mixture. Repeat and lower the speed for a few moments until all is incorporated, scraping down the sides of the bowl as you go.
Fold in the marshmallows by hand.
Using a tablespoon, roll the cookie dough into balls (weighing about 30 g (1 oz) each). Place these balls on the prepared baking sheets, leaving 5 cm (2 inches) between them.
Bake the cookies for 11 to 13 minutes. Cool for five minutes before removing to a wire rack to cool completely.