Just the name pretty much sums it up & says it all.
Either unsweetened or only minimally sweetened Dark Chocolate (sugar-content often no more than 15 to 20% &, inversely, a cacao-content 80%, 85%, & higher). For example, a bar bearing a 100% Cacaolabel equals pure unadulterated chocolate with absolutely no additives (no sugar, no vanilla, no lecithin, etc.). Bars of this range exhibit thick-coated, even pasty texture & surreally smooth melts. Flavors trend toward the extreme; very bold & robust (what some might characterize as strident), often deeply earthen, rarely with high-fruited accents. In short, advanced / challenging material.
These are the nuclear warheads of the chocolate arsenal that literally explode in the mouth. They’re what you call on when needing to summon courage; gain some emergency energy in the middle-of-the-night; impress in the company of others; or just a private well-deserved reward in those moments of what Edmund Burke calls the ‘unbought grace of life’.
Good Brut Chocolate rocks it… hard, with after-shocks… & reverberating memory.