Preheat the oven to 190°C. Line a muffin tin with 9 muffin cups; set aside.
Place the butter into a large heatproof bowl, and melt in the microwave. Whisk lightly.
Sieve the granulated sugar directly into the melted butter and use a whisk to combine.
Grate the lemon zest straight into the mixture. Whisk together.
Pour the buttermilk into the bowl; whisk to combine.
Crack the egg into the bowl - pray no shell goes in - and whisk well to combine.
Sieve the baking powder, bicarbonate of soda, plain flour, and cornflour directly into the mixture. Use a rubber spatula to fold the dry ingredients gently through the wet. Stir in the blueberries.
Divide the batter evenly between the lined muffin cups. Sprinkle 1 teaspoon of demerara sugar on top of each one - don't skimp!
Bake in the preheated oven for 22-25 minutes, or until crisp on top and a skewer inserted into the centre comes out clean. Leave to cool slightly before eating.