by Sarah-Kim Watchorn
Preheat the oven to 180°C/350°F/Gas Mark 4.
Beat the butter with the sugars in a free-standing mixer on a high speed, just enough to bring them together.
Stir the flour, salt, baking powder and bicarbonate of soda together in a medium sized bowl. Add a little to the butter mixture, then slowly incorporate the egg.
Reduce the speed to low and add the rest of the flour mixture along with the chocolate and marshmallows. Mix slowly to combine.
Divide the mixture into 10 even pieces, shape into balls, arranging on a tray lined with baking paper as you go making sure they have enough space to expand between each cookie, you may need two trays here.
Freeze for at least an hour or until ready to eat.
Bake in the hot oven for 12 to 15 minutes until risen and golden in colour. Leave to cool on the tray for a few minutes before moving to a wire rack to cool completely.