Preheat oven to 180 C / Gas 4. Grease a shallow tin, making sure to get into the corners.
In a saucepan over a low heat, melt the butter and add the sugar; stir until the sugar has dissolved. Remove from the heat and stir in the oats. Transfer the oat mixture to the prepared tin and press into the corners.
Bake in the oven for 10 to 15 minutes until flapjack turns golden brown. Remove from the oven and transfer to a cooling rack.
Once cooled completely, cut it into squares and serve.