Preheat the oven to 180°C, fan 160°C, gas 4. Line a 16 x 25cm baking tin with baking parchment.
Melt the butter in a pan over a medium heat. Stir in the maple syrup and sugar, and simmer until it’s mostly dissolved. Remove from the heat and stir in the porridge oats.
Spoon into the tin and bake for 25 minutes. Remove from the oven and score into 16 pieces using a knife. Leave to cool in the tin.
Cook’s tip: if you like your flapjacks to be a little more crisp, leave them in the oven for an extra 5 minutes.