Preheat your oven to 180℃ (or 160℃ for fan ovens) and line an 8x8 tin with baking paper.
Add the butter, sugar and syrup to a large saucepan. Place over low heat and stir gently until the sugar has dissolved and the butter has fully melted.
Remove from the heat and stir in the oats.
Press half of the mixture into your lined tin. Melt the Nutella for 10-15 seconds until runny and spread it over the flapjack. Then top with the other half of the mixture and press it down firmly into place.
Bake for around 20 minutes until golden around the edges. It'll be a little wobbly in the middle at first but don't worry, it will firm up as it cools.
Let the flapjack cool in the tin for around 5 minutes then use a knife to gently score lines where you plan to cut it. Leave to cool completely, around 3-4 hours.
Once cool, remove the flapjack from the tin, cut it into slices using your score marks as a guide and enjoy!