Preheat the oven to 350ºF and prepare three 6-inch or two 8-inch cake pans by spraying the sides with cooking spray and fitting the bottoms with a wax paper or parchment cake circle.

Place all of the dry ingredients into the bowl of a stand mixer and stir on low for 30 seconds to fully combine them. Add the vegetable oil, eggs, vanilla, and buttermilk and mix on low until just combined. With the mixer still on low, add the hot water in a slow stream, then turn the mixer to medium and beat until smooth, about 2 minutes. The batter will be very thin.

Pour into prepared cake pans no more than ⅔ full and bake for 32-36 minutes, until a toothpick inserted comes out clean. Cool completely before filling and frosting.

Pour the heavy whipping cream into a small saucepan set over medium heat and bring to a simmer, whisking frequently. Meanwhile, place the Cadbury Mini Eggs in a glass bowl and microwave them in 30 second increments, until the shells crack under slight pressure and the chocolate inside looks melty. In my experience it took 2.5 intervals (so, 1 minute 15 seconds).

When the heavy whipping cream starts to simmer, pour it over the Cadbury Mini Eggs and cover the bowl with a plate for 2 minutes. Afterwards, whisk the mixture into a uniform consistency, breaking down the shells as you go. It will look pretty rough at first, but I promise it will come together in 1-2 minutes of whisking. When it's fully combined it will look like smooth milk chocolate ganache with tiny bits of candy shell throughout.

Place a piece of plastic wrap over the surface of the ganache and place in the refrigerator for 1-2 hours, until completely cooled and the consistency is much thicker, almost like Nutella.

With a hand mixer or paddle attachment on your stand mixer, cream the butter on medium-high until it’s creamy and light (almost white) in color. About 7 minutes.

With the mixer on low, add the powdered sugar a few cups at a time, scraping down after each addition and making sure each is fully incorporated before adding the next one.

Add vanilla, milk, and salt and mix on low for another minute until fully incorporated.

Once the chocolate cake layers have cooled completely, level and torte each layer to your desired height. Place the first layer on a turntable and pipe a circle of vanilla buttercream around the edge. Fill in the center with half of the (room temperature) Cadbury Mini Egg ganache. Here's a detailed tutorial on this filling method in case this is your first time working with a soft filling.

Place the next cake layer on top and repeat the process of piping the buttercream circle and filling in the center with the rest of the Cadbury Mini Egg ganache. Place the final layer on top, upside down so that the bottom of the cake layer is the top of the cake. Crumb coat the cake with vanilla buttercream and place it in the refrigerator for at least 20 minutes to let everything set up.

Color the Buttercream: Divide the rest of the vanilla buttercream between three bowls and make sure one of the bowls has about ½ Cup more buttercream than the other two. Use food color gels to tint one bowl pastel pink, one bowl pastel yellow, and the bowl with the most buttercream periwinkle blue.

Create the Ombrè Finish: Add each buttercream color into a piping bag and snip off the end. Starting at the bottom of the cake and working your way up, pipe the yellow buttercream while you turn the turntable until it reaches about ⅓ of the way up the cake. Continue on with the pink buttercream until you reach ⅔ of the way up the cake. Finish with the blue buttercream for the top third of the cake and the very top. Use an icing smoother to create a smooth buttercream finish with the ombrè colors, then place the cake into the refrigerator for 20 minutes to let the buttercream set.

Make the Nests: Place the remaining colored buttercream into a bowl and add 2 tsp of cocoa powder on top. Mix until you have a uniform chocolate buttercream and add it into a piping bag fitted with Wilton Tip 4B. Pipe 6 nests on top of the cake. I basically piped a circle with another circle on top. Add 3 Cadbury Mini Eggs into each nest.