Preheat oven to 160 C fan/ 180 C and lightly grease an 8-inch square baking tin. I use silicone, but line your tin with baking paper if it has a tendency to stick.
Combine butter, sugar and golden syrup in a saucepan and heat, gently, over a medium heat, stirring occasionally. Allow to simmer for two minutes and remove from the heat.
Stir in the muesli, pumpkin seeds and ground cinnamon, if using.
Spoon into the prepared tin and press down until the top is smooth. I find the empty butter wrapper or a piece of buttered greaseproof paper helps.
Bake for 15-20 minutes, until lightly golden and set.
Leave to cool for ten minutes before lightly scoring the bars about 1/2 inch deep into 12 flapjacks. This will make it easier to cut once they're completely cool.
When the flapjacks are completely cool, remove them from the pan and cut into 12 bars. Stores well in an airtight container or ziplock bag for up to 10 days, if they last that long!