Stuffed Ebelskivers

1 1/4 cups all-purpose flour 2 tablespoons firmly-packed light brown sugar 1/2 teaspoon baking powder 1/2 teaspoon baking soda 1/4 teaspoon fine salt 2 eggs, separated 3/4 cup whole milk 1/4 cup sour cream 2 tablespoons cold unsalted butter, cut into 14 pieces Fillings: raspberries, semi-sweet chocolate chips, fruit preserves or jam Confectioners' sugar, for dusting Warm maple syrup, for serving