Grease side and bottom of 8 1/2- inch springform pan. Line bottom of pan with a parchment paper circle, grease top of circle. Wrap bottom and side of pan with foil. In top of double boiler over simmering water, combine chocolate, cream, butter, sugar, Grand Marnier and vanilla, stirring until smooth. Remove chocolate mixture from heat; set aside to cool. Stir eggs into chocolate mixture until well blended. Pour batter into prepared pan. Place springform pan in large roasting pan; add enough water to come 1/4-inch up side of springform pan. Bake in a preheated 350 F oven until edge is set but the center is still soft (about 30-35 minutes). Remove springform pan from roasting pan; remove foil. Cool cake completely on wire rack. Wrap with plastic wrap and refrigerate overnight.