Just a heavy mash with some chocolate malt and crystal, brittish yeast BRT101 (RIP Alzymologist Oy, but I still have the Library). Takes time to mature, couple weeks after bottling it still asks for more bottle aging (hopefully few years) but already nice and mellow. Dangerously drinkable, for its ABV.
10 kg into 15 L, that's a malt-head's dream brew :)
I'm at the initial dreaming state of building a 'kuurna', the preferred sahti mashing process. That would be the way to optimise utilisation. I already have a stainless steel piece that would probably work as a base. No use building it though, no room in the house to set up the process or really even store it...