Line a 20cm square cake tin with clingfilm. Put the peanut butter, butter and marshmallows in a large saucepan. Warm over a low heat, stirring until melted. Remove the pan from the heat.
Add the oats, dried blueberries, peanuts and cornflakes to the melted butter mix in the pan, and mix well until combined.
Spoon the mixture into the lined tin, and press down to flatten. Chill in the fridge for 1 hour until set. Cut into 16 squares before serving. If transporting (for lunchboxes or to a picnic), wrap the squares tightly in foil.
Cook's tip: Don't have a baking tin to hand? As these squares are not baked, you could use a similar size plastic container, or spoon into mini cupcake cases, put on a tray and set in the fridge as above.