This drink was mentioned in a thread in the BSL forum. I decided to try making it, not really having any idea what to expect.
It took me awhile to find some Holland Gin, also known as jenever/genever, but I found some. Bols Genever, based on a recipe dating back to 1664, and revised in 1820. Cost me about $35. This is pretty much the "original" gin, before it took on its more modern forms of London Dry, Plymouth and Old Tom. It's a very different gin, with a lot of body. It still has the juniper and other botanicals, but the core flavor is richer, deeper, more like a rice wine or sake.
For the beer, I really wasn't sure what to use. "Dark Lager" is pretty vague. It could be everything from the flavor profile of Spaten Optimator, to Aventinus Eisbock, to Shiner Bock, to a Schwarzbier.
So, I went with the second option... Dublin Stout. Guinness.
Proportion-wise, I found no more information about that drink other than the above description. I kept coming back to the same quote without finding any other sources.
I looked at other beer/liquor cocktails, both modern and more historic. For the most part, they seem to stick with 1oz liquor for every 8oz beer. 2oz for 16, etc.
The recipe I used:
12 oz Guinness
1.5 oz Bols Genever
1 egg yolk
sugar (to taste)
I was expecting this to taste bad, or somehow "weird", but it's surprisingly smooth and well balanced. It's pretty rich, even though it doesn't have a lot of flavor. Everything just blends together really well.
While the original recipe calls for "plenty of sugar", I first tried it without any. Then I added 1 tsp of white sugar and mixed it in. I actually prefer it without sugar; I think the sugar makes it overly sweet, since the drink itself is already fairly malty.
I recommend giving it a try. Just, drink it slowly, since it's plenty strong. I'll definitely be making this again.