Preheat 375 degrees F. Lightly grease a 10-cup/9-inch springform pan with the tablespoon of butter.
Pour the chocolate into a medium bowl. In a small saucepan, set over medium-high heat, melt the butter until it’s very hot and melty. Immediately pour the hot melted butter over the chocolate. Let stand for a minute and then mix until smooth (if there are a few small unmelted chocolate chunks, no biggie). Mix in the 3/4 cup of sugar, cardamom, pure vanilla extract, salt and 4 large egg yolks.
In the bowl of a stand-up mixer (with the whisk attachment), add the egg whites. Beat until frothy, about 1 to 2 minutes. With the mixer going, pour in the 2 tablespoons of sugar and beat for an additional 2 to 3 minutes, until stiff peaks form. Add the beaten egg whites to the cake batter; gently fold them in until thoroughly incorporated.
Pour the cake batter into the prepared springform pan. Bake for 20 to 25 minutes, until a skewer inserted comes out mostly clean (little bits of batter are ok). Transfer to a cooling rack and allow to come to room temperature, about 1 hour. Remove the cake from the spring form pan and serve with a sprinkling of powdered sugar.