Preheat oven to 160°C (140°C fan-forced). Grease a 26cm bundt pan with butter, chill and grease again (this will ensure the cake doesn’t stick to the pan).
Place the sugars and butter in a stand mixer fitted with the paddle attachment and beat until thick and pale. With the motor running, add eggs, 1 at a time, beating well after each addition until completely combined. With the motor running, add the sour cream.
Sieve all the remaining dry ingredients, except the dark chocolate, into a bowl. Reduce stand mixer speed to low, then alternately add the stout and flour mixture, one-quarter at a time, beating until the mixture is smooth.
Add the batter to the prepared pan, smoothing top with a spatula and bake for 1 hour to 1 hour 10 minutes or until a skewer inserted into cake comes out clean. Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely. Meanwhile, for the whiskey butterscotch, place the sugar in a saucepan over medium-high heat. Once a caramel starts to form, swirl the pan so that an even caramel forms. Once a dark caramel has formed, quickly add the butter and remove from the heat. Stir until smooth, then add the cream and set aside to cool completely. Once cooled, stir through the whiskey.
Drizzle some of butterscotch whiskey over the cake, then scatter with chopped dark chocolate. Serve with remaining butterscotch whiskey.