Preheat oven to 350 degrees F. Liberally grease a 24-cavity miniature muffin pan with cooking spray and set aside. In a large bowl, combine the cake mix, egg and oil until a soft dough has formed.

Spoon rounded Tablespoonfuls of dough into each greased muffin cup; gently press down with your fingers. Bake for approx. 10-14 minutes or until the tops appear set and the edges are light golden brown. Allow the cups to cool in the pan for about 5 minutes. Then using the handle of a wooden spoon, make an indentation in the center of the brownie cup. Place a Cadbury Creme Mini Egg inside. Allow the brownie cups to cool before gently popping them out of the muffin cups. If needed, run the tip of a butter knife around the edges to help ease them out.

Once brownie cups have cooled, make your frosting. In the bowl of a stand mixer, cream together the butter and vanilla on medium speed for about 2 minutes, or until light and fluffy. Add in the powdered sugar, about a cup at a time, until frosting is light and fluffy. If needed, add in the cream/milk to help thin it out if it's too thick. Spoon about half of the mixture into a separate bowl and add the yellow food coloring, tinting to your desired shade.

Spoon the yellow frosting on one side and the white on the opposite side of a large, disposable piping bag attached with a large, open-star tip. Pipe high onto cooled brownie bites. Garnish with sprinkles. Serve immediately, or store leftovers airtight at room temperature for a couple days.