Open the oysters, catching the juice as you go in a measuring cup until you reach ½ cup. Place the oyster juice in a saucepan, heating until it boils. Add the champagne and the shallot to the sauce and let it reduce to half. Let the sauce cool until it is lukewarm, then add the egg yolks to the sauce and whisk the mixture well. Place aluminum foil on a baking sheet, loosen the oysters from their shells and fixate the oysters on the foil to make sure they keep their position. Pour the sauce on the oysters and broil the oysters for 3 minutes. Take the oysters out of the oven and serve immediately.