Place the rhubarb and granulated sugar in a saucepan with a splash of water. Cook gently for 5-10 minutes until the rhubarb softens but some of the chunks are still visible.
Put the muscovado sugar, golden syrup, vanilla extract and butter in a large bowl and melt in the microwave. This took 2 minutes in my microwave until fully melted.
Place the oats in another bowl and pour the butter and sugar mixture over it. Conbine well.
Line a square baking tin with baking paper and then spread two thirds of the oat mixture into the base of it. Press down well.
Spread the cooled rhubarb mixture over the oats and then place the remaining third of the oat mixture over the top, again pressing down well.
Bake at 180 degrees Celsius for 35 minutes, until golden on top.
Remove from the oven and allow to cool. The flapjacks will harden as they cool so cut into nine pieces while warm, leave to cool in the tray, and then remove and pull apart once cold.