Preheat the oven to 350 degrees and butter the bottom and sides of a 9-inch springform pan.
Gently melt the butter and chocolate together in a bowl over a double boiler, stirring occasionally. You can also melt them in the microwave at half power in 15 second intervals, stirring after each. Let cool slightly.
Whisk together the egg yolks and ½ cup sugar until light and frothy.
Add the melted butter and chocolate mixture, espresso powder, cardamom, vanilla and salt, then whisk to combine.
In a separate bowl, use an electric mixer or stand mixer to whisk the egg whites and the remaining ½ cup sugar until they form stiff peaks (when they stand straight up and don't collapse), about 3-5 minutes.
Use a rubber spatula to gently fold the egg whites into the chocolate mixture, being careful not to let it deflate.
Bake for about 40 minutes, or until puffed and no longer jiggly in the middle.
Allow to cool completely before cutting.
Dust with powdered sugar if desired and serve cold or at room temperature