Preheat the oven to 425℉/200℃. Line a 12 hole muffin tin.
In a large bowl, combine the flour, sugar and baking powder. Make a well, and pour in the egg, vanilla, butter (make sure it has cooled a little), yoghurt and milk. Stir gently with a wooden spoon until just combined.
Add in the blueberries and stir again. Spoon the batter into the lined muffin tin, filling to near the top of the liners. Add a teaspoon blueberry jam to the top of each and swirl a little with the end of a spoon/chopstick/knife or similar! Optional: Finish with a sprinkle of demerara sugar.
Bake for 5 minutes, then turn the oven down to 350℉/170℃ while keeping the muffins in the oven, and bake for a further 20 minutes, until golden on top. Let cool on a wire rack (this prevents them getting oily on the bottom) - these are especially wonderful eaten warm!
