Pre-heat the oven to 180°C and line a square brownie tin with greaseproof paper. Place 200g of the chocolate, all of the butter, and the orange zest in a heatproof bowl and melt in the microwave, using full power for 30 seconds at a time then stirring and repeating. Be careful that the chocolate does not get too hot otherwise it will burn.
Use an electric whisk, or a stand mixer with the whisk attachment, to beat together the eggs and sugar until pale and fluffy. Leave the chocolate mixture to cool for at least five minutes before stirring into the egg mix, then mix in the flour and cocoa. Stir in the remaining 100g of chopped chocolate, as well as the chopped watercress, then pour the batter into the prepared tin. Bake in the centre of the preheated oven for 35-40 minutes, using a skewer or knife to test if the brownies are cooked. Once the skewer comes out clean, remove the brownies from the oven and allow to cool completely.
To make the ganache topping, gently bring the double cream to the boil in a saucepan. Keep an eye on the cream as it will easily boil over once it starts to simmer. Remove from the heat, then stir in the chopped chocolate, continuing to stir until melted and mixed thoroughly. Finally stir in the Cointreau if using.
Allow the ganache to cool slightly (but not too much or it will harden), then pour over the brownies while they are still in the tin. Smooth the topping out using a pallet knife or spatula, then leave for 30 minutes for ganache to cool and harden before cutting into squares.
The brownies can be kept for 3-5 days in an airtight tin or frozen for up to three months.