Break the Marie Biscuits into small pieces in a bowl.
Beat the eggs and set aside.
In a large microwaveable dish, melt the butter in the microwave, then add the cocoa, icing sugar, vanilla essence and eggs. Mix well.
Cook the chocolate mixture in the microwave for 2 minutes on high.
Add biscuit pieces to the chocolate mixture and mix until the biscuits are covered in chocolate.
Spread the mixture on a Swiss-roll pan lined with baking paper.
While the brownies cool slightly, prepare your icing by mixing the butter and cocoa powder together. Add in the milk and icing sugar and beat until smooth, about 2 minutes. Spread on the icing.
Leave in the fridge overnight or until set (about 2 hours).
Cut into 4cm squares.
