Freeze the mini creme eggs for at least 1 hour.
Add the sugar and eggs to a bowl and use a hand mixer to beat them well until you have a pale yellow colour and they triple their volume.
In a pan set over a low to medium heat, add the milk and butter cut into cubes, and heat them up until the butter is melted.
Pour the milk mixture over the egg mixture whisking really well.
Sift in the flour, cocoa powder, baking powder, and the vanilla extract, and mix well to get a runny batter.
Line 12-hole muffin tray with non-stick muffin cases, and add one tablespoon of batter to each case.
Add one mini creme egg to the cases, then cover them with another tablespoon of batter so they are completely covered.
Bake it the preheated oven at 180 degrees Celsius (350 Fahrenheit) for approximately 20 minutes or until well risen and firm to touch.
Leave to cool completely.
To make the chocolate buttercream, add all the ingredients to a food processor and blitz until smooth.
Use a piping bag fitted with the nozzle of your choice to decorate each cupcake with buttercream.