Preheat oven to 350°F.
Liberally butter bundt pan and dust with flour, knocking out excess.
Bring Guinness Stout and molasses to a boil in a large saucepan and remove from heat. Stir in baking soda. Cool to room temperature.
Sift together flour, baking powder, and spices in a large bowl. In separate bowl; whisk together eggs and sugars. Whisk in oil, then molasses mixture. Add to flour mixture and stir until just combined.
Pour batter into bundt pan and tap pan on counter to eliminate air bubbles. Bake in middle of oven until a tester comes out with just a few moist crumbs approx. 45 to 50 minutes. Cool cake in pan on a rack 5 minutes then turn out onto rack and cool completely.
Dust cake with confectioners sugar- serve with whipped cream.