Preheat the oven to 175°C (approximately 350°F). Rinse the cherries and remove the pits. Rinse the lime in hot water, dry, finely zest and squeeze the juice. Double the paper cups and spread on a greased baking sheet.
Mix the flour and baking powder. Beat the butter, sugar, vanilla sugar and egg until creamy.
Fold the lime zest, 2 tablespoons of lime juice and the kefir into the butter mixture. Quickly mix in the flour mixture. Fold in the cherries. Pour the batter into the paper cups. Bake for 20-25 minutes, until golden brown. Brush the muffins with the remaining lime juice and let cool. Serve dusted with powdered sugar.