Crack the eggs into a bowl. Set a timer to 15 minutes and start beating the eggs at medium speed. Halfway through the mixing (about 7 minutes in) gradually add the sugar. Keep mixing until the timer rings - this recipe needs long mixing so go for a total of 15 minutes.
When the timer rings stop the mixer. Combine the flour and the baking powder and add it to the egg mixture one tablespoon at a time, mixing at low speed now.
Transfer the batter into a muffin tin lined with paper cups. The cake will grow, so do not overfill. Depending on the size of the pan the final amount of cupcakes might differ from 12. Bake in the preheated oven at 180°C for 20 minutes, or until a toothpick inserted in the centre comes out clean. As the cupcakes bake you can prepare the topping.
Add 100 ml of milk to a saucepan and set it on medium heat.
In a separate container combine the remaining ingredients: egg yolk, sugar, cornstarch and 60 ml of milk. Blead with a hand blender to combine.
When the milk has warmed up, pour in the egg mixture and bring to a boil stirring continuously. The custard will thicken quite quickly. Remove from heat when it boils. Cool to room temperature then add to a piping bag.
Combine the butter, cut in chunks, and sugar and mix at medium speed with a hand mixer for about 5 minutes, until creamy and fluffy. Stir in some cocoa powder to colour the buttercream brown. Transfer to a piping bag with a "spaghetti" tip.
Make sure that the cupcakes have cooled to room temperature. Pipe a circle of buttercream around the edges of each cupcake to create the outer edge of the nest. Pipe the custard inside the buttercream nest. Pipe a smaller circle of buttercream to make the top part of the nest. Place three chocolate eggs inside each nest, setting them in the custard.
Store refrigerated. Best served at room temperature.