Preheat oven to 350°F (175°C). Grease two 8-inch pans and line bottoms with parchment.
Mix and bake cake batter per box directions, 25–30 minutes, until a toothpick comes out clean. Cool completely.
Trim one cake into a dome by carving a rounded top for the mushroom cap. Leave the other flat for the base.
Stack cakes with a thin layer of frosting between. Crumb coat with frosting and chill 15 minutes.
Tint remaining frosting red. Frost the dome smoothly, then press white fondant circles onto cap for spots.
Roll black fondant into a small rectangle for the face and press on. Pipe a rim of white frosting where cap meets stalk.