Preheat the oven to 170C. Line the 24x24cm tin with baking parchment.
Place the butter, sugar and golden syrup in a pan and put it on low heat. Stir once in a white until the butter has melted and sugar has dissolved. Don't let the butter bubble.
In the meantime, mix the oats, flour and dried cranberries together in a large bowl.
Add the butter mixture to the dry ingredients and mix well until there're no dry ingredients left. Tip the oat mix into your lined baking tray and press down firmly.
Bake for 25 minutes on the middle shelf until the edges of the flapjack are starting to colour.
Leave to cool completely in a the baking tray.
When your flapjack is cool, melt your white chocolate in a microwave (in periods of 30 seconds, stirring in between, until melted) or in a bowl over a pan of simmering water.
You can either spread white chocolate on your flapjacks with the back of the spoon, use a piping bag for an intricate pattern or simply trickle it erratically all over.
Wait for the chocolate to set before cutting your flapjack into 16 squares.