Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Melt the butter, sugar and golden syrup over low heat on the stove. When the sugar has dissolved, remove it from the heat and let it cool for a minute or two.
Add the oats to a large bowl then pour in the wet mixture. Mix well until the oats are evenly coated.
Press the mixture firmly into your tin and bake for around 20 minutes. The flapjacks should be golden at the edges and a bit wobbly in the middle. They will firm up as they cool.
Leave the flapjack to cool at room temperature for about 3 hours, then cut into slices and enjoy!
Notes
Store in an airtight container in the fridge for up to 1 week.