Preheat the oven to 180 C. In a large bowl, cream together the butter and brown sugar until light and fluffy. Beat in the egg yolks then stir in the vanilla. Stir in the flour and mix well.
In a shallow dish, such as a pie plate, beat egg whites until foamy. Spread walnuts on a plate or greaseproof paper.
Roll 1 tablespoon of the biscuit dough into a ball; roll in egg whites then in walnuts. Place 5cm apart on a baking tray. Make a depression in each ball with your thumb.
Bake in the preheated oven for 12 to 15 minutes.
When the biscuits have cooled, melt the chocolate in the microwave. Spoon just a bit of the melted chocolate in the hollows of the biscuits, then place 2 to 3 mini eggs (depending on their size) into the hollows. Allow to cool long enough so that the chocolate hardens and the eggs are secure in the nests.