Line baking tin with greasproof paper and preheat the oven to gas mark 4 / 350°F / 117°C
Place the can of Guinness into a saucepan and bring to a boil. Then, allow it to simmer until it reduces approx 2/3 of the amount. This can take around 20 minutes to do. Then set aside and allow to cool. You will be using 114ml of the Guinness in the Brownies.
Create a double boiler / ban-Marie / Place a mixing bowl over a pan of water and place int he butter and chocolate. Allow them to melt together until smooth and remove from the heat.
Next, add in the sugar and whisk together, followed by the 114ml of Guinness until fully combined. Follow this by adding in the eggs, again mixing in fully. Sift in the flour, folding this into the mixture and finally add in the vanilla extract and salt. The brownie batter should look shiny, thick and smooth.
Pour into the prepared baking tin, and tap the tin gently a few times. Place into the oven and bake for approx 30 minutes, checking if the brownie is baked. Use a skewer in the middle and if it comes out clean, then the brownie is ready. Be careful as the brownie bakes very quickly towards the end!
Remove from the oven and allow to cool fully before adding on the Baileys ganache topping.