My absolute favorite bakery in the area is Froggy's. Their carry-out store, next door to the French restaurant, has a complete range of cakes, as you can see on their website. The quality is awesome - including real buttercream. They sell complete cakes as well as individual slices, in case you would like to try some in advance of ordering a full cake.
Gourmet Frog
316 Green Bay Rd
Highwood, IL 60040
847-433-7038
Tuesdays-Saturdays 9am-6pm
froggysbakery.comTwo other high-quality nearby bakeries I like are
Three Tarts in Northfield, and
Gerhard's in Lake Forest. But since you're looking for real buttercream, my top recommendation is Froggy's, all the way.
(And I'll second - no, third - the advice to avoid Deerfield Bakery.)