Preheat the oven to 180°C. Cut the baking paper into squares that fit comfortably in the cavities of the muffin tin, the ends should protrude considerably. Once you add the batter, the baking paper will stay put.
Squeeze the lime. Mix the caster sugar, flour, baking powder, 1 pinch of salt and vanilla sugar in a bowl with a whisk. In another bowl, mix the butter, yoghurt, sunflower oil and eggs with a whisk.
Pour the wet ingredients into the dry ingredients and mix into a batter with the whisk. Add the lime juice and blueberries. Fold through the batter. Divide the batter evenly between the paper sheets in the cavities of the muffin tin. Bake the muffins in the middle of the oven for about 20 minutes, until golden brown and done.
Tip: Scrub the lime, zest the green skin before you squeeze the juice. Add the zest to the batter to give the muffins an extra zingy flavour accent.
