Preheat nonstick pan over low. Add butter and chocolate (broken/cut into pieces) to pan and melt together. Remove from heat.
Separate egg whites from yolks and add to separate bowls. Whip egg whites to soft peaks. I find that hand whipping creates a better result than using a mixer, but either method works. Set aside.
Into the yolks, add buttermilk, sugar, cornstarch, salt, vanilla, coffee, and the chocolate/butter mixture. Stir until combined, then fold in egg whites.
Add mixture to a well oiled 9" springform pan (with a round of parchment at the bottom). A brownie pan also works if you don't have a round.
Bake in preheated 275°F/135°C oven for 50-60 minutes.
When finished, it should still be slightly jiggly in the center. Resist the urge to cut this warm! Allow to cool fully before cutting.
Garnish with a dusting of dark cocoa powder, a dollop of whipped cream, toasted hazelnuts, a drizzle of nice olive oil, and a pinch of flaky salt.
WHIPPED SOUR CREAM:
Whisk cream and sugar together into soft peaks, about 3-4 minutes. Add sour cream. Whisk to incorporate.