Preheat oven to 180°C (Gas mark 4/350°F). Line a 12 hole muffin tin with 12 muffin cases.
Place the butter or dairy free spread into a mixing bowl and whisk together with the caster sugar until light and fluffy. Break the eggs into a bowl and lightly whisk with a fork. Add the eggs a little at a time to the butter and sugar mix, whisking in between.
Sift the flour and cocoa powder on top, and using an electric whisk on a low speed, gently blend the ingredients together. Add the milk and vanilla extract and blend again. Increase the mixer speed and whisk for a few more seconds until well mixed.
Divide the cake mixture between the 12 cupcake cases and smooth over the tops with the back of a spoon. Bake for 20-25 minutes until risen and springy to the touch.
Once the cakes are cooked, transfer to a wire rack to cool. Whilst the cakes are cooling, make the chocolate buttercream icing:
Whisk the butter with an electric whisk until light and fluffy. Sift in the icing sugar and cocoa powder, a little at a time, until completely combined.
Add the vanilla extract and whisk again. Add the milk in small splashes until the icing is at the correct consistency for piping.
Using a palette knife, spread a layer of buttercream icing over the top of each cupcake.
Spoon the remaining buttercream into a piping bag with a grass piping nozzle. Pipe a nest shape around the edge of each cupcake.
Finish off each cupcake by adding 3 mini egg chicks to each nest.