Good for beginners, the piped buttercream daisy decoration on this cake requires only two pastry tips and no complicated curves. Scoring the cake first gives it graphic appeal and makes it easy to space the flowers evenly.
Trim the top of the cake with a long serrated knife to make the surface level. Using a ruler as a guide, insert toothpicks or wooden skewers into the sides of the cake at 2-inch intervals to mark two equal layers. Rest the serrated knife on the toothpicks, and halve the cake horizontally using a sawing motion. Carefully slide the top cake layer onto a cardboard round, and set it aside.
With a small offset spatula, spread top of bottom layer with 1 1/4 cups frosting; carefully slide the second cake layer back on top of the first layer.
Fit a pastry bag with a coupler, and fill with frosting (for petals); set aside. Tint 3/4 cup of remaining frosting dark green (for leaves and stems) and 1/3 cup frosting bright yellow (for dots); set aside. Tint 4 cups frosting chartreuse (for base coat) by mixing leaf green, lemon yellow, and a touch of egg yellow.
Gently brush away loose crumbs from top and sides of cake with a pastry brush. With the offset spatula, spread about 1 1/2 cups chartreuse frosting over top and sides of cake to form a crumb coat. Refrigerate cake until frosting is firm, about 15 minutes.
With a large offset spatula, spread about 2 more cups chartreuse frosting over top and sides to form a second coat. Smooth top and remove excess frosting with the large offset spatula. Smooth sides with a bench scraper. Return cake to refrigerator; chill until second coat is firm, about 15 minutes.
Lightly score surface of cake with a knife to mark 8 wedges, wiping knife clean after making each mark.
Decorate cake: Using green frosting and the #3 tip, pipe a slightly curved line for the stem. The daisy's petals are made with the #352 leaf tip. Using white frosting and holding the bag with the tip's pointed end facing up, apply pressure and pull out straight from the stem; release pressure and pull upward to finish each petal. Repeat, for twelve or more petals, wiping off the tip between strokes if necessary. Switch to yellow frosting and the #3 tip. Holding the bag at a 90 degree angle to the cake, make a mound of tiny dots in the center. Finally, with green frosting and the #352 leaf tip, pipe a small leaf originating at the base of the stem (using the same technique described for the petals).
Refrigerate cake until ready to serve. Let stand at room temperature 20 minutes before serving. Slice into wedges: To avoid crumbs on surface, make each cut with one downward motion, pulling knife back toward you (not upward) and wiping knife clean after each cut.
Cake can be refrigerated for up to four hours.