Any American bakers on here?

[JellySlice]

Sometimes I see pictures or videos where they sprinkle a liquid on the sponge layers before buttercreaming. What is the liquid?

Simple syrup (flavoured or not) to keep the cake moist?

Just sugar & water boiled together? And how much?

Could you sprinkle, say, elderflower cordial, or would it have to be diluted?

Like a lemon drizzle cake?

I think elderflower cordial would be delicious!