7.

For the Crumb Coat: Level cakes with a serrated knife (full directions here) and set scraps aside for snacking. Place 1 layer on a heavy cast iron turntable. If you like, a waxed cardboard cake round can first be placed underneath, secured to the turntable with a scrap of damp paper towel. Top cake layer with exactly 1 cup frosting, using an offset spatula to spread it evenly from edge to edge. Repeat with second and third layers, then cover sides of cake with another cup of frosting, spreading it as smoothly as you can (tutorial here). Refrigerate cake until frosting hardens, about 30 minutes, and repeat if desired for a thicker coat. Meanwhile, prepare the ganache as directed.