In a large sauce pan mix Guinness and chocolate chips over medium heat. Use a wood spoon to evenly combine the melted chocolate with the stout.
Remove both pans from heat and add melted chocolate and Guinness mix to roux and combine thoroughly.
Add remaining ingredients and mix together. Save some of the walnuts for later. Pour into a 9 inch round cake tin or similar cake pan. Top the cake mix with remaining walnuts. Bake for 45 minutes at 350° or until center is cooked.
In a sauce pan melt the butter for the icing. Remove from heat then add cocoa, powdered sugar and vanilla extract. Gently whisk until smooth.
When chocolate sauce has cooled, add egg and whisk until thickened. Pour icing on top of Guinness Chocolate Walnut Cake and serve.
How to make a roux: Start by melting a some butter in a pan. Stir in a little bit of all-purpose flour. Continue to stir flour into the butter. Cook it for a few minutes so that you don’t have that raw flour flavor. This can be a lot more to it depending on how fancy you want to get but, you know, don’t sweat it.
Butter the bottom and sides of a 9 inch square pan and line the bottom with parchment paper. Dust with cocoa powder and tap out the excess.
In a heatproof bowl, combine the butter and both chocolates. Create a double boiler by setting the bowl over a pot of simmering water, stirring occasionally until it's melted. Remove the bowl from the heat.
PRO-TIP: You do not need a lot of water for this. Just enough so it creates steam and so it does not all boil away.
In a stand mixer fitted with your whip attachment, whisk together the egg yolks and 1/2 cup of sugar on high speed until it goes pale, increases in volume and it leaves a defined ribbon on the surface when you lift the whisk (about 5 minutes).
Fold in the melted chocolate until fully incorporated.
In a clean and dry bowl, whisk the egg whites and salt on a low speed until the form soft, wet peaks. Gradually beat in the remaining 1/2 cup of sugar and continue whisking at high speed until the whites form stiff peaks.
Scrape the batter into the pan and gently smooth the top.
Bake for about 35 minutes (It will still have a slight wobble on the top). Look for the cake to rise and formed a cracked top.
Let cool for 15 mins.
Invert the Chocolate Pave onto a plate or a cake board and remove the parchment paper. Re-invert onto your serving plate. Dust with icing sugar if you wish.