Grease and line a 22cm square baking tin with baking parchment.
Into a saucepan over a medium heat, combine the butter, sugar and syrup and stir until melted. Remove from the heat and stir in the oats and chocolate chips. Spoon into the prepared baking tin and press to compact the mixture with the back of a spoon.
Bake in the preheated oven for 20 to 25 minutes until golden brown. Remove from the oven and make cuts for the flapjacks (I make 16 little ones).
Melt the 100g chocolate in a double boiler or in a pan over barely simmering water then pour on top of the flapjack. Go over the cuts you made again, as otherwise they are hard to cut once cold! Allow to cool for about 20 minutes.
Once it's cooled stick it in the fridge for a bit, 20 to 40 minutes, till you can't resist and have to have one or two or three. Remove from the fridge, lift out of the tin using the baking parchment to help you then cut into squares and serve.
You may want to adjust the volume of oats - I like my flapjacks quite moist, but if you want them harder, go up to 350g.