Preheat your oven to 375°F. Line your cupcake pans with cupcake liners and set them aside.
Mix the cocoa powder into the boiling water, allow it to sit for 2 minutes.
In a medium mixing bowl, whisk together the flour, sugar, and salt set it aside.
In a large mixing bowl, whisk together the egg, oil, and buttermilk. Stir the flour mixture into the egg mixture.
Pour the cocoa powder minute into the batter and stir just until combined.
Fill the cupcake pans so they are halfway filled; the batter will be on the thin side. Unwrap the Cadbury eggs and insert an egg into the center of each cupcake.
Bake at 375°F for 5 minutes, then turn the oven down to 350°F for 12-15 minutes, until a toothpick can be inserted into the center of a cupcake and come out clean.
Allow the cupcakes to cool while you make your buttercream.