Put the butter and the chocolate in a medium-sized heatproof bowl and set over a pan of simmering water, stirring occasionally until melted. Alternatively, cover the bowl loosely with cling film and microwave at 30 sec intervals until melted. Leave the melted chocolate mixture to cool to room temperature.
Heat oven to 180C/160C fan/gas 4. Grease a 20cm square tin with butter and line with 2 long strips of folded baking paper or foil – one running top to bottom, the other left to right then line the base with a piece of baking paper. The strips will help you lift it out later.
Break the eggs into a large bowl and tip in the caster sugar. With an electric mixer on maximum speed, whisk the eggs and sugar for around 5-8 mins or until they look thick and creamy and have doubled in size.
Pour the cooled chocolate mixture over the egg mixture, then gently fold together.
Sieve the flour and cocoa into the wet ingredients then continue to fold the mixture gently with a spatula just until everything is fully combined, try not to over-mix.
Pour the mixture into the prepared tin and carefully level the mixture with the spatula. Put in the middle of the oven and bake for 20 minutes.
Meanwhile, cut the fondant filled eggs in half and set aside then place the mini eggs into a pestle and mortar. Crush a few of the mini eggs but leave some whole. After 20 mins take the brownie out of the oven and press in the fondant eggs, cut side up. Scatter over half of the mini eggs too then put it back in the oven for a further 5 mins.
Once baked, top with the remaining mini eggs, leave to cool completely then place in the fridge for about 1hr to firm up. Lift it out of the tin using the strips of baking paper and cut into squares to serve. Decorate with little Easter chicks if you like.