Prepare the Cake Batter
Preheat oven to 350°F (175°C) and grease three 9-inch round cake pans.
Prepare the cake mix according to package instructions.
Divide the batter into four bowls and color each portion with a different gel food coloring.
Bake the Cake Layers
Pour each colored batter into separate cake pans and spread evenly.
Bake for 10-12 minutes, or until a toothpick inserted into the center comes out clean.
Let the cakes cool completely before cutting.
Cut the Mini Cake Rounds
Use a 2.5-inch round cutter to cut out circles from each cake layer.
You should get about 21 cake rounds (for 7 mini cakes with 3 layers each).
Make the Frosting
Beat softened butter until fluffy.
Gradually add powdered sugar, heavy cream, vanilla extract, and salt. Beat until smooth.
Assemble the Mini Cakes
Layer the cake rounds with a thin layer of frosting in between.
Frost the top and sides of each cake and smooth with a spatula.
Decorate and Serve
Top each cake with Cadbury Mini Eggs and sprinkles.
Chill for 15 minutes before serving for the best texture.