How-to Fill and Frost a Layered Cake
After the cake layers have completely cooled to room temperature, use a soft pastry brush to carefully remove any loose crumbs on the top and sides of each layer. Trim off any crisp edges with a knife or kitchen.
To keep crumbs from loosening from the cake and mixing into the frosting, first spread a very thin layer of frosting over the top and sides. If the frosting is very thick, it may need to be thinned with a teaspoon or two of milk or cream.
To keep the serving plate clean, line it with 3-in. strips of waxed paper. Center the first cake layer over the strips. To use frosting as the cake filling, dollop about 1/2 cup frosting over the top of the cake and spread to the edges. Repeat with the second layer, if frosting a three-layer cake.
If your cake filling is different from the frosting, it might spill out the edges and discolor the frosting. To prevent this, pipe or spread a 1/2-in. strip of icing around the top edge of each layer before spreading the filling in the center. This keeps the filling inside.
Top with the last layer. Frost the sides of the cake, building up the top edge of the cake slightly. Make large swirling strokes in a vertical.
Spread remaining frosting over the top of the cake, smoothing right to the edges of the cake. Swirl top to match sides. Garnish or decorated if desired. Carefully remove waxed paper by pulling one piece at a time out from under the cake.