Preheat the oven to 325°F/163°C (with the fan or convection setting turned off)
Grease and line an 8-inch square baking pan.
Melt the butter in a pan over medium-low 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 melted butter and the mixture resembles melted caramel.
Add the oats and mix until well coated.
Spread into the prepared pan and press evenly with the back of a spoon or spatula.
Bake until they start to firm. This will take 20–30 minutes, use the shorter cooking time for more chewy flapjacks and longer if you prefer them crisper. The flapjacks will firm and crisp more as they cool.
Store covered at room temperature for up to 3 days.
PLEASE NOTE:The Metric measurements are not a true conversion. These 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).The number of servings will vary depending on what size you cut the flapjacks into.
In the US, I recommend using quick cook oats, but if old fashion is all you have, cut the measurement down to 2 cups, and increase the golden syrup to 6 tablespoons.
Lining your pan with baking paper will make it easier to remove the flapjacks from the pan.
Can’t find golden syrup? Use dark corn syrup in its place.
When melting the butter and sugar together, cook over low heat. Higher heat can end up hard and taffy-like.
To avoid a greasy flapjack, mix the butter and sugar mixture well until you see no sign of melted butter.
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.
For a more cake-like, fluffy flapjack, skip the stovetop and instead, add all the ingredients to a food processor, blitz it all together, and spread the mixture into the pan. Then bake as directed.
If you don’t like a crisp edge, try using a damp cake wrap (paid link). This will cool down the sides and stop the edges from browning too quickly.
Update: When I first published this recipe, I had some users that thought they were too crisp, so I updated the recipe with instructions on how to make them to your liking instead of my preference.