Can this icing go on a sponge cake? Be under a chocolate collar?
Yes and yes
You’ve referred to it as a meringue, but I just wanted to point you in the right direction: it’s a buttercream and compared to a meringue is more dense. Compared to other buttercreams it has a custard undertone to the flavor and a bit of lightness. The caramel and its variations (choc praline, burnt orange) are my all-time favorite buttercreams, with the possible exception of the white chocolate vanilla bean buttercream in Rose’s Heavenly Cakes (Golden Dream wedding cake).