Preheat oven to 160 degrees celsius (fan-forced) and line a 20cm square tin with baking paper. Set aside.
Place the butter into the Thermomix bowl. Melt for 3 minutes, 80 degrees, Speed 2.
Add the caster sugar and cocoa powder and mix for 10 seconds, Speed 4. Scrape down the sides of the bowl and repeat for a further 10 seconds. Allow to cool slightly.
Add the vanilla extract and the eggs and mix for 10 seconds, Reverse, Speed 4. Scrape down the sides and repeat.
Add the plain flour and mix for 5 seconds, Speed 4. Scrape the sides of the bowl and repeat. Stir through the mini solid Easter eggs with the spatula.
Pour the brownie mixture into the prepared tin and bake for 25-30 minutes or until cooked through. Leave in the tin to cool completely.
Allow to cool and then cut into slices. Store in an airtight container at room temperature for up to 5 days.