Preheat the oven to 160°C / 140°C fan assisted / gas mark 3 / 320°F.
Line and grease a 20cm (8 inch) square cake tin.
Weigh out 75g of the buckwheat flakes and 50g of the flattened rice flakes and tip them into a food processor. Pulse them gently so they break down into more of a fine flour.
Toss the flour into a large mixing bowl with the rest of the buckwheat flakes, flattened rice flakes, the ground cinnamon and the salt. Set aside for a moment.
Melt the butter in a large pan over a low heat then add the golden syrup and sugar.
Once the sugar has melted and the ingredients are well combined, remove from the heat.
Pour the dry ingredients into the syrup mixture, stirring well until the dry ingredients are completely covered and coated in the mixture.
Pour the flapjack mixture into the tin and press down evenly and firmly.
Bake in the oven for 45-50 minutes.
Remove from the oven and leave to cool in the tin.
Once cool, remove from the tin and cut into squares.