Preheat the oven to 200C/180C Fan/Gas 6. Line an 8″ Square baking tin.
In a medium saucepan combine the butter, brown sugar, golden syrup and chocolate and heat on low until the butter has melted. Turn off the heat and stir in the oats and hazelnuts.
Pour the oaty mixture into the baking tin and press down. Bake for 20-25 minutes until golden brown at the edges (it's fine if still a little wobbly in the centre). Let cool on a wire rack.
Once cooled, slice into 16 squares. Loosen the Nutella by microwaving for about 30 seconds. Use a teaspoon to drizzle over the flapjack. Let set then serve!