Grease and line a 30x12 cm baking tray with baking paper. Preheat the oven to 170℃/340 F.
Add the butter, sugar and syrup to a large saucepan over low heat, stirring often .
Mix everything nicely. Once butter has melted , add condensed milk and vanilla extract. Cook, stirring for further 5 minutes until it turns a shade darker.
Remove from the heat and gradually stir in the oats. Mix thoroughly until they are nicely coated.
Pour the mixture into the prepared baking tray . Using the back of a spoon or a spatula, flatten the mixture into the tin evenly, making sure you press down evenly and reaches into corners.
Bake in the preheated oven for 18-20 minutes or until the edges of the flapjacks are beginning to turn golden. The middle of the flapjacks will be pale and wobbly . It will set as it cools . Remove from the oven.
(I like to sprinkle some chocolate chips on top when there in 1 min left to bake. When you take the flapjacks out , the chocolate chips will be soft and you can easily spread them with a help of a butter knife if you like chocolatey flapjacks, or just drizzle some melted chocolate on top once they are done. This step is completely optional. )
Leave in the tin to cool completely for 30 minutes . Then take out of the baking tin, still in baking paper and cut into squares. Make sure it is completely cold before cutting into slices.
Enjoy!