I'm getting ready to cover a small 4" round cake to look like a coffee mug. I'd like it to have a lip so I can put ganache inside of it and have it look like an actual cup full of coffee...but I don't know how I should actually cover it to get this effect. Can anyone help, please?

Hmmm ok...maybe roll out a thin piece of fondant or some really stiff butter cream (like a dam to keep filling in) and put this on the top of your cake around the edge and cover with fondant....

HTH

Or you can scoop a circle of cake out, leaving a rim of whatever size you like....

HTH

The cake scooping circle would've been much better. For future reference, a loop of fondant underneath didn't work so well...just got lost in the buttercream.