Preheat your oven to 170°C. Grease a 20cm square baking tin (or a small rectangular tin) and line the base with baking paper.
Beat the eggs and castor sugar together for a few minutes, until pale and fluffy. Beat in the melted butter and vanilla extract.
Sift in the flour and cocoa and fold into the mixture. Put four Romany Creams to one side, you will be using these to decorate the brownies later. Cut the remaining biscuits into quarters and stir them into the brownie mixture.
Tip the brownie mixture into the prepared tin. Bake for 25 minutes but keep an eye on the brownies towards the end. They should be set on top with a thin crust, but be a bit soft in the middle. Set the brownies aside until they are completely cool and have set.
To prepare the icing: sift the icing sugar and cocoa into a small bowl. Add the melted butter. Dissolve the coffee granules in the boiling water and add this to the bowl. Stir until you have a smooth icing, it should be quite thick but still spreadable. Add more or less water if necessary.
Place the four Romany Creams you set aside earlier into a small sandwich / ziplock bag. Use a rolling pin to crush them up gently. Spread the icing over the cooled brownie slab. Sprinkle immediately with biscuit crumbs then cut into squares.