Butter a 9 or 10-inch springform pan and line it with parchment paper.
Pour the stout into a large saucepan, slice up the butter, and add to the beer. Heat over medium heat until the butter melts. Whisk in the cocoa and sugar, then set aside to cool briefly. In a small bowl, whisk together the flours and baking soda.
In a large bowl, beat the sour cream with the eggs and vanilla. Pour in the beer, then stir in the flour mixture until incorporated.
Pour the batter into the prepared tin. Preheat oven to 350℉ and let batter rest while the oven heats. Then place in the oven and bake for about 1 hour, checking to see if a tester inserted in the center comes out clean, the top is firm, and the edges of the cake are starting to come away from the sides of the pan. Remove from oven and let cool for 20 minutes in the pan, then turn out onto a rack to cool completely. I like to chill my cake in the fridge before frosting.
Make Frosting
In a large bowl, whip the cream cheese with an electric mixer. Sift the powdered sugar over and beat to incorporate. Then beat in the heavy cream.
Frost the cake with swirls of frosting. Nigella says: "Ice the top of the black cake so that it resembles the frothy top of the famous pint."
Notes
Bring ingredients to room temperature. Bring the ingredients to room temperature before you incorporate them into the batter. This is especially important for the cream cheese—you do NOT want cold cream cheese, which can create a lumpy icing!
Hydrate the batter. Make sure to let the batter rest while the oven heats. Gluten-free flours benefit from a rest before baking so that the batter can hydrate.
Sift the powdered sugar. Again, you want a beautifully smooth frosting! Sift the powdered sugar into the bowl when you make the frosting to eliminate any lumps.
Frost the cake in swirls. When adding the frosting, use an offset spatula to create smooth swirls. Nigella says: "Ice the top of the black cake so that it resembles the frothy top of the famous pint."
Use gluten-free flour: If you wish, substitute 2 cups of 1:1 gluten-free flour such as Bob's Red Mill or King Arthur Flour for the oat and sweet rice flour, or use my homemade gluten free flour blend.
Stout substitute: If you are OK with gluten beer, Guinness is the obvious choice. You could also try a non-alcoholic gluten-free stout if you don't drink alcohol. Another option is tea, but I think if you can't do any kind of beer it's probably best to make another cake recipe.
Make dairy-free: Substitute your favorite non-dairy butter and sour cream for the butter and sour cream. Substitute non-dairy cream cheese for the cream cheese and non-dairy milk for the heavy cream.
Make refined sugar-free: Substitute coconut sugar for the granulated sugar in the cake.