Melt together the butter, sugar and golden syrup in a pan over low heat.
In a large bowl, stir the cocoa powder into the oats.
Once the butter, sugar and golden syrup is all melted together and smooth, pour it over the oats in the bowl and mix well until it is all combined.
Line your slow cooker pot with a couple of layers of baking paper (one to lift the flapjacks out with, one to protect from burning). I used an oval 3.5 litre slow cooker for this recipe.
Press the mixture into the pot, so it is an even depth and roughly smooth.
Place a tea towel or some kitchen rolls squares under the lid to catch drips and put on the lid.
Cook on high for two hours or until the edges have started to firm and the centre of the flapjacks bubble slightly.
After two hours, put your broken up chocolate on top of the flapjack mixture and cook on high for another half an hour, so the chocolate melts in slightly.
Remove from the pot using the baking paper edges to pull it out carefully, then place on a cooling rack and allow to cool and become firm for 2-3 hours.
Once cool and firm, slice into 12 pieces.
I made this recipe in an oval 3.5 litre slow cooker. If you have a larger slow cooker you may wish to double the recipe so it is not too thin. If you have a round slow cooker then your mixture will be thicker too.