Why not get someone to bake the cake for you, go out to a fancy restaurant and have your nice meal and then when you get home, each get a champagne glass and dig into the cake at the same time for your gender reveal? I mean you said it’s only for you two so no big deal where you do the cake bit.
Get coloured cocktails/drinks?
Can you bring your own cake and they can just serve it?
Cute idea. I have not seen that. You could try asking them yo write the gender on the desert plate? Like they will for birthdays and such? It may be hard to get a restaurant to do colored cake if it's not a normal menu item.