Preheat the oven to 400 degrees F.
Option 1- Grease a 12-portion nonstick muffin tin with melted butter or non-stick spray.
Option 2- Line the muffin tin with paper baking cups.
Cut circles from the puff pastry sheets using a 3 1/2-inch plain cutter and transfer them to the cups in the muffin tin. Press the dough evenly into the edges of each muffin cup, lining the sides.
Whisk the milk, confectioners' sugar, egg yolks and egg together in a large glass measuring cup until silky smooth.
Pour the mixture into each crust, leaving 1/8 inch at the top. Bake in the center of the oven, approx 12 to 15 minutes, until the pastry begins to brown. The filling should be set but still jiggle and a toothpick inserted into the center of a tart comes out clean.
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.