Preheat oven to 190 degrees celsius (fan forced) and grease and line a square baking tin with baking paper. Set aside.
Place the butter, dark chocolate and brown sugar into the Thermomix bowl and melt on 50 degrees, Speed 2, 4 minutes (or until melted). Allow to cool slightly.
Add the eggs and vanilla extract and mix on Speed 4, 8 seconds.
Add the plain flour, salt and cocoa powder and mix on Speed 4, 5 seconds. Scrape down the sides of the bowl and repeat for a further 5 seconds.
Stir through the mini solid Easter eggs with the spatula.
Place into the oven to cook for 25 minutes or until a skewer inserted into the middle comes out slightly sticky with crumbs.
Allow to cool and then cut into slices. Store in an airtight container at room temperature for up to 5 days.