Pre-heat the oven to 160c / 140 Fan / Gas 3. Prepare the brownie tin. Grease and fully line the brownie tin with greaseproof paper, ensuring there is excess paper beyond the edge of the tin to aid the removal of the brownie once cooked.
Melt the butter and chocolate. Cube the butter and place into the bowl with the broken chocolate. Set the bowl over a pan containing some water. Ensure the base of the bowl doesn’t touch the water in the pan. Place the pan on the hob over a low heat. Allow the butter and chocolate to slowly melt, stirring periodically with a spatula. Once the mixture has completely melted remove the bowl from the pan. Sit the bowl on a clean cloth / kitchen roll to dry the underside. Allow the melted chocolate to cool for a few minutes.
Meanwhile prepare the coffee. Spoon the coffee granules into a cup and add the boiling water straight from the kettle. Stir until dissolved. Set aside to cool.
Mix the eggs and sugar. Place the sugar and eggs (including the extra egg yolk) into a large mixing bowl. Use a balloon whisk to mix until well combined.
Combine the two mixture. Pour the melted chocolate and butter mixture onto the combined eggs and sugar. Mix together well.
Finish the brownie batter. Add the prepared coffee and mix well. Sieve the flour into the bowl and fold it in with a spatula. Add most of the walnuts to the batter and mix until just combined.
Fill the brownie tin. Pour the batter into the prepared brownie tin. Scatter over the reserved walnuts. Bake. Place the brownie tin in the centre of the oven and bake for 40 – 45 minutes. You may need to rotate the tin after about 25 minutes of baking.
Cool. Pour some water into the base of a large roasting tin. You’re aiming for it to be about 1 – 2cm deep. Remove the cooked brownie from the oven and sit the brownie tin (containing the brownie) into the tray carefully so as to avoid splashing the brownie.
Slice. Once fully cooled use the excess greaseproof paper to lift the brownie from the tin and sit it on a board. Slice into 9 equal pieces. Use a pallet knife to remove the slices from the greaseproof paper.