Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Melt the butter over low heat with the sugar and syrup. Mix gently until the sugar has dissolved.
Add the oats and cinnamon. Mix well until the oats are evenly coated.
Press the mixture firmly into your tin and bake for around 18-20 minutes. The flapjacks should be golden at the edges and will be a bit wobbly in the middle until they start to cool.
After 5 minutes, gently score the flapjack where you plan to cut it later.
Leave the flapjacks to cool at room temperature for 3-4 hours, then cut into slices using your score marks as a guide.
Video
Notes
Store in an airtight container in the fridge for up to 1 week.