Preheat the oven to 170°C/gas 3. Lightly butter a 23cm-square baking tin and line the base with baking parchment.
Put the butter, syrup and sugar together in a saucepan over a low heat, stirring occasionally until the butter and sugar are melted. Put the oats, flour and cranberries in a large bowl and add the butter mixture. Mix thoroughly. Tip the mixture into the prepared tin and press down. Bake for 25-30 minutes, then allow to cool completely. Then cut into squares and remove from the tin.
Meanwhile, melt the white chocolate in a bowl over a pan of simmering water. Spread molten chocolate carefully on each flapjack, leave to set, then serve.
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