Preheat the oven to 190°C (170°C fan oven, 375°F, gas mark 5). Lightly grease a 12 cup muffin tin.
To make your Lemon Duffin, put the butter and sugar in a bowl and whisk together until light and creamy. Whisk in the egg and Lemon Extract.
Sift the flour and Baking Powder on top, and mix together, gradually adding all the milk, to make a smooth, thick, batter.
Divide the Duffins mixture between the muffin tins, smooth the tops and bake in the oven for 30-35 minutes until risen to a dome and firm to the touch.
As soon as the Duffins are cooked, turn them onto a wire rack. Put the lemon curd into a piping bag fitted with a 1cm (1/2inch) wide plain metal nozzle and inject each one through the side with the lemon curd.
To decorate, while the Duffins are still warm, put the sugar on a plate. Working on one Duffin at a time, brush all over with melted butter and roll in the sugar to coat. Put back on the wire rack. Your Lemon Duffins are now ready to serve warm or allow to cool completely.