Thirsty’s response is perfectly reasonable. I don’t know whether the OP is male or not. However, I’m male, and to say that I’d be wary of offering to take a 20 year old intern out for dinner to “celebrate her birthday” would be putting it mildly. They say you “live and learn”, and I’ve certainly learned from the last eight times I’ve done it.
In all seriousness, personally I’d get a €210 One-For-All voucher and present it to her in front of people to avoid any confusion. People often bemoan the fact that vouchers are not personal, but it’s hard to buy a personal gift for someone who you don’t really know, and there’s often an element of forcing one’s own taste on someone when you choose a gift for them.