Peanut butter rice crisps.

Grease and line an 18cm shallow square tin.
Melt 50g butter in a pan. Add 180g mini marshmallows and heat, stirring, until melted.
Remove from the heat and stir in 100g peanut butter until evenly mixed. Add 75g Rice crispies (or a store's own brand) and stir gently until they're all coated.
Put in the tin and level the top. Leave until cold, then cut into squares.

Makes 16

Per serving:

Calories - 116
Fat - 5.9g 
Sat Fat - 2.4g