@pavel94—
I've not had it, but it can be bought from Amazon and elsewhere, and I found a recipe: [ul]Collect acorns from a white oak or from a red oak, with the former being preferrable; avoid those that have obviously been penetrated by worms.
Leave the acorns in a grocery bag for a week or more, until they turn brown.
Discard any that now show signs of infestation.
Boil the acorns for 30 minutes.
Shell and peel the acorns, again discarding those that show signs of infestation.
Grind the acorns in a food processor.
Especially if you used acorns from a red oak, boil the pieces again, and then run cool water over them until the water is clear.
Roast the acorns at 400°F (200°C) on the lowest rack, stirring occasionally, until they blacken (about 35 minutes).
Grind the acorns in a coffee grinder.
Use one heaping tablespoon for every 6 ounces of water. (So a bit less than 12 times as much water by volume as grounds.) [/ul]