Frost cake, making the sides and top as smooth as you can. Transfer to the freezer to set for 10 minutes.
Divide the remaining buttercream into three bowls.
To make the light brown fur color: add a drop of yellow gel and black food gel. Add little by little. Mix until smooth.
To make the black fur color: add 3 to 5 drops of gel food coloring. Mix until pitch black.
Leave the last bowl of plain, uncolored buttercream alone. We're gonna use that for the bottom half of the face.
Draw the top part of the face, creating a V shape. Use the guide/photo above for help. Fill it in and smooth the buttercream around.
Repeat with the bottom part of the face using the white buttercream. Transfer to the freezer for 10 minutes, until set. Remove it from the freezer and smooth out the surface using your hand or a spatula.
Add the nose by plopping on some frosting so it's nice and high and pronounced. Pipe on the eyes using the black frosting. And transfer back to the freezer for about 10 minutes.
Meanwhile, create the ears by taking the marzipan and adding a drop of yellow and black to the center. Mix the marzipan together until the color turns brown. Roll it out into a 1/2-inch thickness and cut out, using a knife, two triangles. Smooth out the edges using your fingers. Add a toothpick to each of the ears.
Slice the airhead into a 3-inch strip. Round the end of the 3-inch strip to resemble a tongue. Using a knife, score a line down the center of the "tongue."
Remove the cake from the freezer and smooth out the nose and eyes (CAREFULLY!). Pipe on the mouth, creating a U shape under the nose. Set the "tongue" right under the nose. Add the ears to the side of the cake.
Transfer to the freezer for an additional 5 to 10 minutes to set.