Start by preheating your oven to 180°c (160°c if you're using a fan-assisted oven or set to Gas Mark 4). Prepare an 11x7 inch baking tray by greasing it with some leftover butter or margarine and then line it with parchment paper.

Place a large saucepan on the stove and combine the butter, golden syrup, and brown sugar. Heat them gently, stirring often until they are fully melted and amalgamated. Alternatively, you can melt these ingredients in a microwave-safe dish, making sure to stir frequently to avoid any hot spots.

In a separate large bowl, combine the oats and cinnamon, tossing lightly to mix. Pour the melted butter mixture over the oats and stir until every oat is evenly coated.

Pour the oat mixture into your greased and lined tray. Use a spatula to spread the mixture evenly into the corners of the tray.

Bake the mixture in your preheated oven for 22-25 minutes. The flapjacks should be golden around the edges while still slightly soft in the center with a bit of a wobble. It might appear underdone, but it will firm up as it cools.

Once removed from the oven, if the edges have puffed up, you can press them down gently with the back of a cold spoon to flatten them slightly.

Allow the flapjack to cool completely in the tray. Once cool, use the edges of the parchment paper to lift the flapjack out of the tray. Peel back the paper gently. If the center is sticky or soft, carefully use a thin spatula to help lift the flapjack without breaking.

Cut the flapjack into your preferred size pieces and serve.