Preheat oven to 325 degrees (F). Grease a 9-by-2-inch springform pan.
In a large microwave-safe bowl, put the chocolate, butter, and salt. Melt in the microwave for 90 seconds. Stir and microwave again, in 1-minute intervals, until completely melted.
In a separate bowl, beat the eggs and sugar with a mixer until light and thickened, about 8 minutes with a handheld mixer or 4 minutes with a stand mixer.
Fold the melted chocolate into the whipped eggs until evenly combined.
Pour the batter into the springform pan and bake about 1 hour and 25 minutes, until a toothpick inserted into the cake comes out damp but not runny.
Remove cake from the oven and cool on a rack.
Once the cake is cool, carefully remove from the pan, invert onto a serving platter, and sift a light layer of confectioners' sugar over the top.