Preheat the oven to 160˚C fan / 180˚C / 320˚F convection / 356˚F.
Line a 20 x 20cm (8 x 8") square baking tin with baking paper / parchment.
In a ban marie (a heatproof bowl set over a pan of barely simmering water), melt the dark chocolate and butter. Cool.
Using a stand mixer fitted with the whisk attachment, or an electric whisk, whisk the eggs and caster sugar together for 3-5 minutes until light and doubled in volume.
Gently stir the cooled melted butter and chocolate into the fluffy beaten eggs and sugar.
Sift in the plain flour, salt and cocoa powder and stir with a spoon until just combined.
Gently fold in 40g (1½ oz) mini eggs and 60g (2oz) roughly chopped milk chocolate.
Pour the mixture into the prepared tin and knock gently to release any air bubbles.
Place in the preheated oven and bake for 20-23 minutes, until the brownie doesn't wobble in the middle, and a skewer inserted into the middle of the brownie comes out sticky not raw.
(If the skewer comes out very sticky and covered in runny batter, return to the oven for 2 minutes and test again. Repeat if need be.)
Take the brownie out of the oven and whilst still warm sprinkle over the remaining mini eggs, gently pressing them in slightly.
Leave to cool in the tin. Cool completely before cutting in to squares.
Store in an airtight container at room temperature for 5-7 days, or freeze for up to 1 month.