For the cake:
Preheat oven to 350°F. Grease 3 (9-inch) round cake pans and line with parchment paper. Grease again or spray with non-stick cooking spray. Set aside. Place all ingredients in a large bowl. With an electric mixer, stir on low speed to combine (30 seconds). Then turn mixer up to medium and beat for 2 minutes. Divide batter evenly into the 3 prepared cake pans. Bake in preheated oven for 25-30 minutes or until a toothpick stuck in the middle of the cake comes out clean. Let cool.
For the frosting
Beat softened butter on medium speed with an electric or stand mixer. Beat for 3 minutes until smooth and creamy. Add powdered sugar, cream, and vanilla extract with the mixer running on low. Increase to high speed and beat for 3 minutes.
To decorate cake, fill and frost cake with a thin layer of frosting. Place in refrigerator to chill for 1-2 hours. Meanwhile, divide the remaining frosting into 5 separate bowls. Add food coloring to each - dark pink, light pink, purple, blue and green. Prepare 5 pastry bags with the following pastry tips: 4B (blue). 32 (purple), 1M (light pink), Russian pastry tip (dark pink), and use a coupler for the green frosting, so you can use 2 different pasty tips - #3 and #352. Take the cake out of the fridge and prepare the ganache. Place the chocolate chips and oil in a microwave safe bowl. Heat on high for 30 seconds. Stir. Repeat. Stir until smooth. Add more oil if it’s too thick. Spread on top of the cake and push over the edges with a spoon to create the drizzles. Let it set in the refrigerator for 20 minutes. Next, pipe the flowers and leaves on top of the cake. Use the green frosting with the #3 tip to write the birthday message.
Note: This recipe makes more frosting then you will actually need for this cake but it's nice to have extra to practice your piping skills or add more flowers if you wish.