We came up with this cake for my sister's birthday. It took less than a batch of fondant and two cake mixes. I did two 9-inch circle pans and a heart pan.
I covered the cakes with homemade frosting (1/2 cup Crisco, 2 cups powdered sugar, 1/2 tsp vanilla, 2 Tablespoons milk) right before I added the fondant. This helps it stick to the cake.
To make the fondant, melt one 16 oz bag of mini marshmallows with 2 Tablespoons water in a large bowl. Melt on 30 second intervals, stirring after each round of 30 seconds...about 1 1/2 minutes total.
Add 3 cups of powdered sugar. Stir until smooth. Put Crisco on the counter. Knead in about 3 more cups of powdered sugar. (There were about 2 cups of powdered sugar left in the 2 lb. bag.)
Put Crisco around the ball of dough. Cover with Saran wrap. Place in an air-tight container until ready to work with it.
I split the fondant into three piles. I colored one purple, one blue (a little bit of blue food gel turned it the teal color), and left the other pile white.
I just rolled out the fondant and cut strips and circles for the outside decorations. I rolled balls out of the fondant for the bottom and cut out the letters by hand.