Preheat the oven to 350° and spray two 8-inch cake pans with nonstick spray. Add a parchment round to the bottom of the pans and spray the parchment with the nonstick spray. Set aside.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
Add the buttermilk, lemon juice, lemon zest, vanilla, and oil to the dry ingredients and beat with the paddle attachment of a stand mixer or a hand mixer just until fully combined.
In a separate bowl, whip the egg whites until they reach medium-stiff peaks. Fold the egg whites carefully into the batter just until the streaks of egg whites have been absorbed.
Separate the batter evenly into the two cake pans and bake for 35-40 minutes, or until the center of the cake springs back when pressed on. Let cool completely. I find the cake is easiest to frost when it has been chilled, so wrap each cake in a couple of layers of plastic wrap and store in the fridge for at least 2 hours or overnight.
While the cake is baking prepare the lemon curd by whisking the egg yolks, sugar, lemon juice, lemon zest and salt together in a medium saucepan. Heat over medium heat, whisking constantly, for about 3-4 minutes, until the mixture starts to thicken.
Add in the butter and whisk until it is all melted. Continue to cook until the mixture if very thick and coats the back of a spoon. Because this is intended to fill a cake, you want to make sure the curd is quite thick and not runny.
Remove from the heat and strain through a fine mesh sieve into a bowl. Press a layer of plastic wrap against the surface of the curd and chill for at least 2 hours, or overnight.
In the bowl of a mixer, beat the butter using the paddle attachment for 3-4 minutes, until it is light in color and fluffy in texture.
Add the powdered sugar, salt, and milk. Mix on low until the powdered sugar is mostly absorbed, then increase the speed to medium and beat for 4-5 minutes, until the buttercream is nice and fluffy.
Tint the frosting a light blue or add 1 drop of green to the blue if you would like more of a light teal color. Set aside until ready to frost the cake.
Place one cake round on your cake plate and using a piping bag with a large round tip, or the end of the bag snipped off, pipe a thick border of buttercream around the top of it to create a well in the center for the lemon curd filling.
Fill the center of the well with lemon curd and smooth. Place the second layer of cake on top. Use the piping bag of buttercream to pipe thick lines of buttercream all around the outside of the cake until it is fully covered.
Use an offset spatula to smooth the sides and tops of the cake, wiping excess buttercream onto the side of a bowl as you go.
In a small bowl combine the cocoa powder and boiling water until the cocoa powder is completely dissolved. Dip the tines of a clean, unused toothbrush into the mixture and use your thumb to flick the tines at the cake. This will splatter the cocoa powder mixture onto the cake and give it that distinct speckled egg look.
Make a pile (a nest!) of toasted coconut on top of the cake (or several smaller nests) and decorate with the chocolate eggs.