Preheat your oven to 325°F (163°C). Prepare an 8-inch springform pan by lining the bottom with parchment paper and greasing the sides generously.

In a heatproof bowl, combine 1 pound of bittersweet or semidark chocolate and 16 tablespoons of unsalted butter, along with ¼ cup of strong black coffee or liqueur, if using. Set the bowl over a pot of simmering water and stir until everything is melted and smooth, about 5-7 minutes.

In a stand mixer, beat 4 cold eggs on high speed for about 5 minutes until they double in volume and become pale and frothy.

Gently fold the whipped eggs into the cooled chocolate mixture in thirds without deflating them.

Pour the combined batter into your prepared springform pan and bake for about 22-25 minutes, or until the cake slightly rises and a thin crust forms on top.

Once baked, allow it to cool to room temperature and then refrigerate for at least 6 hours or overnight.

Before serving, dust the top with confectioners' sugar or unsweetened cocoa and slice with a warm knife.