1. Preheat the oven to 350F, and grease and line a 9 inch springform tin.
2. Pour the stout into a large saucepan, and add the sliced butter.
3. Heat until the butter is melted, and remove the saucepan from the heat. Stir in applesauce.
4. Whisk in the cocoa and sugar.
5. Beat the yogurt with the egg, yolks and vanilla, then pour into the saucepan. Stir well.
6. Stir in the flour and baking soda with a spatula (batter will be thick and dense).
7. Pour the batter into the greased and lined tin, and bake for an hour.
8. Leave to completely cool in the tin - this is very important because it is quite a large, damp cake.