Preheat oven to 180°C fan bake or 200°C regular bake. Grease a 12-hole muffin tin or line with paper cases. Mix flour and Chelsea Caster Sugar together in a large bowl. In a separate bowl, whisk egg, milk and lemon zest together. Add to dry ingredients. Add melted butter and mix until only just combined. Pour into prepared muffin tin and bake for 15-20 minutes. While muffins are cooking, make the drizzle topping by whisking the lemon juice and Chelsea Caster Sugar together until the sugar has dissolved. As soon as the muffins are cooked (they should spring back when lightly pressed) take out of the oven and spoon the lemon drizzle topping all over the tops. Leave the muffins to sit in the tin for approximately 3 minutes before taking out and leaving to cool on a wire rack.