Heat the oven to 180°c and lightly grease a muffin tin.
In a bowl, whisk together the yogurt, egg, sugar and vanilla extract.
Mix in the butter and fold in the flour until mixed well.
Place 1 tbsp of the batter into each muffin hole. Add 1 tsp of the jam into the centre of each and then cover with more batter.
Place the muffins into the oven and allow to bake for 15-20 minutes until golden brown and cooked through.
Remove from the oven and allow to cool for 5 minutes in the tin before removing and rolling in the caster sugar.