Filipino Calamansi Muffins

Method

1. Heat the oven to 190°C (170°C fan) and line a twelve-cup muffin tin. Wash the calamansi before zesting or cutting. Strain out all seeds and measure the juice; a count of fruit alone is unreliable because size and juiciness vary.
2. Whisk flour, sugar, baking powder, baking soda and salt in a large bowl. Make sure no lumps of raising agent remain. In a jug whisk eggs, oil, milk and vanilla, then add calamansi juice and the optional zest just before combining with the flour.
3. Pour the wet mixture into the dry mixture and fold with a spatula until no large pockets of flour remain. Stop while the batter is still slightly lumpy. Do not beat smooth: the acid and raising agents are already reacting and vigorous mixing toughens the crumb.
4. Divide promptly among the twelve liners, about three-quarters full. A scoop helps keep portions even. If the tin is small and you have extra batter, bake a separate small portion rather than filling cups to the rim.
5. Bake for 18–23 minutes until the tops spring back and a skewer comes out without wet batter. Rotate the tin only after the muffins have risen and set. A pale top can still be cooked; do not leave the muffins in until they are dry merely for darker colour.
6. Rest in the tin for 5 minutes, then lift onto a rack so steam does not wet the bases. Cool at least 15 minutes before serving. The citrus aroma is most apparent once the muffins are warm rather than piping hot.
7. Cool completely before packing. Keep covered in a cool room for up to 1 day, or refrigerate for up to 3 days; freeze for longer storage. Warm a portion briefly before eating. Discard if mould develops rather than removing just the visible spot.