Melt the butter in a pan over medium heat. Add the brown sugar, syrup, vanilla and cook until the sugar is dissolved and fluid. Remove from heat and whisk until there is no visible butter and the mixture resembles melted caramel.
Add the oats and mix until well coated.
Spread evenly into the prepared pan and bake 15-20 minutes or until golden.
PLEASE NOTE:The Metric measurements are not a true conversion. They are the measurements needed to make this recipe work with British oats (I developed the recipe using jumbo porridge oats). You can find these measurements by clicking the metric button above the ingredients list).
In the US, I recommend using quick cook oats. If old fashion is all you have, cut the measurement down to 2 cups, and increase the golden syrup to 6 tablespoons.
When melting the butter and sugar together cook over low heat. Higher heat can end up hard and taffy-like.
When baking, watch closely and remove from the oven when the edges are golden. Overcooking may result in a hard, crunchy flapjack.
For a crisper flapjack, use a shallower baking pan and bake at a higher temperature.