This is a great paste to use when trying to fill in cracks or leveling two cake together! I learned about it when I took several cake decorating classes from Chef Toba Garrett. It has proven to be indispensable.
CAKE RECIPE HELP

QUESTION: ….Also, since I don't have the big commercial pans, should I do two of the smaller ones and glue them together? I really need help on this one! Thanks! PAT
SARAH SAYS: Yes, you can bake two smaller pans, and then cut the ends. Place then ends together. I fill in the indent with a “spackle paste.” 

INGREDIENTS
3-4 cups of cake crumbs; use the same crumbs of the cake you'd like to fix
1/2 to 3/4 cups buttercream or frosting used to frost the cake with
corn syrup, if needed

INSTRUCTIONS
1. Place the cake in a large bowl. Add the frosting. Mix all the ingredients to form a thick paste. If the paste is too stiff, add more buttercream to soften, not water. If it is really dry, add 1/8 teaspoon corn syrup to soften; no more.

2. Let dry at room temperature before icing cake.  

STORAGE
Keep stored in an airtight container. Refrigerate if the frosting needs refrigeration.