Preheat the oven to 180°C, 350°F, gas mark 4, line the baking tin with parchment paper and set aside for later.
Pour the butter, golden syrup, light brown sugar and cocoa powder in a large mixing bowl or pan. Melt on a medium to high heat stirring frequently. If you’re melting the ingredients in a microwave, heat in short bursts approximately every 10 to 20 seconds and stir the mixture each time.
Once melted, leave the mixture to cool down for around 10 minutes. Then add the oats, white chocolate chips and milk chocolate chips into the mixing bowl and stir until evenly distributed and the oats are coated in the mixture.
Pour the flapjack mixture into the lined baking tin and press down with the back of the spoon. Make sure the mixture is pressed down evenly and into all four corners. Bake in the oven for 15 to 20 minutes, the flapjacks will slightly brown in the oven. Once baked, leave the chocolate flapjacks to cool.
Break the chocolate into small pieces or cubes and place in a small mixing bowl. Melt in the microwave until melted, microwaving in short bursts, stirring frequently.
Pour the melted chocolate over the flapjacks and spread with an offset spatula. Leave to chill in the fridge or at room temperature for an hour or until the chocolate has hardened.