Butter 4 tartlet molds. Preheat oven to 220°C (approximately 425°F). Rinse grapes and pat dry. Halve and remove seeds from red grapes. Divide green and red grapes among molds. In a metal bowl, whisk yolks with sugar. Place over a pan of hot water and continue whisking until very frothy.
Remove egg yolk mixture from water bath and gradually beat in champagne until mixture is cold. Whip cream until stiff and fold into champagne custard. Divide custard over the grapes and bake on the middle rack of the oven until golden-brown, 10-15 minutes. Serve gratins dusted with powdered sugar.