Preheat oven to 350°F. Line 2 (12-cup) standard-size muffins pans with paper or foil cupcake liners.

Prepare cake batter according to package instructions, and divide evenly among cupcake liners. Bake in preheated oven until a toothpick inserted in center comes out clean, about 20 minutes. Cool cupcakes in pans on wire racks 5 minutes. Remove from pans, and cool on wire racks until completely cool, about 20 minutes.

Meanwhile, beat butter in bowl of a stand mixer fitted with a paddle attachment on medium speed until pale and fluffy, about 5 minutes, scraping bottom and down sides of bowl twice while beating. With mixer running on low speed, gradually add powdered sugar, beating until smooth, about 2 minutes, stopping to scrape down sides of bowl as needed. Beat in vanilla and salt on low speed until incorporated, about 15 seconds. With mixer running on medium speed, gradually add cream, beating until fluffy and spreadable, about 30 seconds. (For best results, beat frosting by hand with a wooden spoon for about 2 minutes to remove air bubbles.)

Stir together 1/2 cup white frosting and cocoa in a small bowl until well combined; set aside. Stir together 1/4 cup white frosting and green food coloring in another small bowl until well combined; set aside. (Add more green food coloring until you reached desired color.) Add yellow food coloring to remaining white frosting, and stir together until combined; set aside. Add more yellow food coloring until you reach desired color.

Transfer chocolate frosting to a piping bag with a 1/2-inch corner snipped off. Transfer green food coloring to a piping bag with 1/2-inch corner snipped off; cut a 1/4-inch slit through center of snipped end of piping bag. (This will create the veins in the leaves.) Transfer yellow frosting to a piping bag with a 1-inch corner snipped off, and cut a 1/2-inch slit through the center of snipped end of piping bag.

Pipe a small round of chocolate frosting in the center on top of each cupcake (about the size of a quarter), and smooth flat with the back of a spoon. For the sunflower petals, start on the outer border of the top of the cupcake (pointing at the edge of the chocolate frosting) with yellow frosting piping bag held at a 30° angle from the cupcake and the slit on the side of the piping bag (facing east and west), and pipe a small mound of frosting, pulling piping bag away from the mound, creating a sharp tip for the petal. Repeat piping going around the border of the cupcake until first row of petals is complete. Repeat same technique for second row, piping each petal in between the petals on the bottom row and overlapping onto the chocolate center. You should have 2 rows of piped petals. For the green leaves, holding the piping bag at a 30° angle from the cupcake, and the slits facing up and down, pipe small mounds between flower petals, and then pulling piping bag away from the mound, creating a sharp tip for the leaf. Repeat on remaining cupcakes.