I either buy cardboard cake rounds from a kitchen supply company or make them myself, from cardboard boxes. (orangette.net)
Secure the first brownie layer to a six - inch cardboard cake circle, or directly to a cake stand, using a small amount of buttercream. (flourcoveredapron.com)
I like to use thick cardboard cake boards which you can also pick up at local cake supply shops. (graciouslysweet.com)