Coat 8 ramekins with butter and then sprinkle with sugar. Preheat oven to 375 degrees F. Add raspberries with 1/2 of the sugar to a food processor and process until smooth. Strain raspberry mixture through a strainer into a bowl. In a separate bowl, beat egg whites until stiff. Gradually add remaining sugar and beat until glossy. Add about 1/4 of egg whites to raspberry puree and fold together until blended. Add remaining egg whites to raspberry mixture and fold together until mixture is even in color. Spoon mixture into ramekins and set them on a baking sheet. Bake for 10-12 minutes or until puffed and just start to turn golden on top. Remove from oven and sprinkle with sifted powdered sugar.