Stir the maple syrup, sugar, bicarbonate of soda and egg yolk into the melted butter then mix in the flour and cinnamon. Beat to a smooth creamy dough.
Drop teaspoons of the mixture, spaced slightly apart, on to baking sheets lined with nonstick baking paper. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 4–5 minutes until golden brown. Leave to harden for 1–2 minutes then loosen and transfer to a wire rack.
Melt the plain dark and the white chocolate in separate heatproof bowls set over saucepans of gently simmering water.
Hold a biscuit over one of the bowls and spoon a little of the chocolate over half of it, spreading it with the back of the spoon. Return the biscuit to the wire rack and coat all the biscuits in the same way, so half are covered with dark and the rest in white chocolate. Leave in a cool place for 30 minutes until the chocolate has hardened. Store in an airtight container, the layers separated with sheets of nonstick or greaseproof paper, for up to 2 days.