I've been meaning to reply to this. My girlfriend made me some wonderful Black & Tan cupcakes earlier this year. (not quite stout cake, I know - but close enough) This is her recipe. Low-sugar, as well - for my diabetic brethren. 142 calories, 5.6g fat, 21.5g carbs, 9g sugar, 3g protein per cake.
Bass (or Harp) Cupcakes:
1/3 C Butter
1/2 C Sugar + 1/2 C Splenda OR 3/4 C Splenda baking blend instead of both
2 Large eggs
1 t vanilla
1 1/2 C Cake flour (not all-purpose, or they'll be too dry)
1 t Baking powder
3/4 t Baking soda
1/2 C cocoa powder
1 C Beer
Beat the butter until smooth, add the sugar(s), beat until fluffy. Add the eggs one at a time, beat until smooth. Add the vanilla. Mix the dry ingredients together and add to the butter sugar egg mixture in two batches, alternating with the beer. Fill cups up half way only. Bake at 350º for 18 minutes.
Guinness Caramel Buttercream Frosting
1 C Diabetisweet brand Brown Sugar flavored sugar substitute
1/2 C water
2 large eggs
3 sticks butter, room temperature, plus 2-3 T in reserve, just in case
1/4 - 1/2 C Guinness
Combine the Diabetisweet and water until dissolved, then cook over medium-low heat to 238ºF - the soft ball stage. Meanwhile, beat the eggs until pale yellow and ribbony. Just before the sugar reaches 238, turn on your stand mixer with the whisk attachment and get those eggs going again, then add the sugar VERY SLOWLY in a thin, steady ribbon while mixing. Keep it mixing until the bottom of your bowl is room temperature; it should be looking fluffy and almost marshmallowy. Leave that mixer going, and add the butter one tablespoon at a time, letting it beat in thoroughly before adding the next tablespoon. You'll know if you're going to fast because the mixture will get soupy if too much butter is added at once - just take your time and if this happens put it in the refrigerator for 10 minutes, then start again. It will probably look "curdled" at one point - a little chunky and separated - just keep mixing mixing mixing until it seems smoother. Once all the butter is added and your frosting looks smooth and spreadable, add the Guinness VERY SLOWLY, pausing frequently to scrape the sides of the bowl. This could destabilize your emulsion and make it look curdled again - if so, add your reserved butter one tablespoon at a time until it looks smooth again. If you're going to use it immediately, refrigerate for 10 minutes first - the added liquid will make the frosting softer than normal buttercream.