You start by melting the butter and chocolate au bain marie. You place a heatproof bowl on a pan of gently boiling water. Put the butter and chocolate in it and let it melt gently. Then stir well until smooth and allow to cool to room temperature.
Preheat the oven to 180˚C. Grease a 20 x 20 cm square baking pan and line with baking paper. Leave a little overhang on the sides so it can be taken out easily.
Put the eggs together with the caster sugar in the bowl of the mixer. Turn the mixer on high and beat until the sugar and eggs have become thick and creamy. This will take about 5 minutes.
Pour your (cooled) chocolate mixture over your egg mixture and gently fold it in.
Now add the flour and cocoa (handy to strain briefly to avoid lumps) and fold this all in again.
Transfer your batter into the prepared baking pan and smooth the top a bit with the spatula.
Now chop your chocolate eggs a bit. Doesn't have to be very fine. They'll melt anyway. Leave a few whole or half. Sprinkle this over the top of your batter. Keep a few to put on top of your brownie (optional).
Place in the oven and bake for about 20-25 minutes or until a wooden skewer comes out dry.