Preheat oven to 350°F. Grease a 9x13-inch pan with spray or butter.
Mix cake batter per box directions. Pour into pan and bake 25–30 minutes or until a toothpick comes out clean. Cool completely.
Smooth chocolate frosting over the top with an offset spatula. Chill 10 minutes to set.
Cut a dinosaur silhouette from cardstock or trace and cut parchment. Place on chilled cake.
Spread green frosting over the whole cake, about 1/4-inch thick. Remove stencil carefully to reveal the silhouette.
Refrigerate 30 minutes before serving so frosting firms.