Grand Marnier is a blend of brandy (cognac) and orange, you can definitely taste the brandy in it. Your choice of liqueur depends on add-ins, are you planning to add melted chocolate or white chocolate or fruit puree/curd?
My first choice to pair with the white/choc butter cakes you are making would probably be the white chocolate version of mousseline, and you could add to that either a little extra vanilla extract, or a liqueur that suits your taste. Grand Marnier would be absolutely lovely, and as Roxanne says, doesn’t end up tasting boozy, especially if you make it a day ahead of serving. If you don’t like brandy you could choose cointreau (pure orange/no brandy), or frangelico (hazelnut).