A bitter black coffee drink, earthy and a bit boozy - it was ok, and you ought to try it if you really like a black cold brew coffee. Flavor driven by the espresso liqueur (I used Borghetti), with booziness boosted by the rum (Plantation OFTD), earthiness accentuated with the stout (Black Marble with nitro widget), and the creme de cacao (Tempus Fugit) adds more sweetness (chocolate notes lost behind the liqueur).

Used a dark german beer and it was downright delicious. Will have to try with an even darker beer next time (i.e. stout of some kind).

I can tell this one will take up too much of my time in brand experimentation... I used a 9% abv Imperial Stout, Appleton SB, and Kamora (terrible, saccharine; Mr. Black is far superior than any other coffee liqueur I've tried, and I'm continually wasting money trying other brands)... The ingredients I employed resulted in a flavorsome but too-sweet swill... I did gas-off the stout (stirring). I can't imagine using Guinness due to its carbonation, but I'll do it for science.

I have had this with both a standard proof Jamaican rum (Appleton) and Irish Whiskey (Jameson) and they were both excellent.
Not sure about the ‘shake all ingredients’. First time with the Guiness was a bit fizzy! I now add it at the end and stir.