Preheat oven to 350°F.
Prepare the bundt pan. Melt one tablespoon of butter, then use a pastry brush to brush the melted butter generously over the inside of a 10-cup bundt pan. (Johnson recommends brushing from the base up to prevent any butter pooling). Scatter over one tablespoon of cocoa powder, moving the pan from side to side to coat it evenly. Then, tap it upside down to remove any excess cocoa. Set aside until ready to use.
Make the cake batter. Add the remaining butter and cocoa powder with the sugar, vanilla bean paste and boiling water to a large mixing bowl and whisk until smooth and the butter has melted and the sugar has dissolved. Pour in the Irish stout, beaten eggs and sour cream and whisk until smooth. Fold in the salt, flour, baking powder and baking soda, then pour into the prepared pan.
Bake for 40–45 minutes or until a skewer inserted into the cake comes out clean. Leave to cool in the pan for 10 minutes on a wire rack, then invert onto the wire rack to cool completely.
To make the sour cream frosting: Beat the butter, sour cream and powdered sugar together until smooth, then spread over the top of the cooled bundt cake. Store for up to three to five days in an airtight container.