Position the rack in the middle of the oven. Preheat the oven to 350°.
Rub The Inside Of An 8-Inch Cake Pan Or A Springform Pan With Butter. Swirl A Few Teaspoons Of Flour Around To Coat Completely.
Invert The Pan And Tap Out The Excess. Combine The Chocolate And Butter In The Top Of A Double Boiler Over Simmering Water. (Or, Alternatively, Combine In A Medium, Heavy Saucepan And Set On A Flame Tamer/Diffuser Over Very Low Heat).
Stir Occasionally Until The Chocolate Is Melted. Set Aside To Cool.
Meanwhile, Sift The Flour, Baking Powder, And Salt Into A Bowl; Whisk Well And Set Aside.
Combine The Eggs, Vanilla Extract In A Large Bowl. Whisk Until Foamy. Add The Sugar And Whisk Until Light And Frothy, 1 Minute.
Blend In The Chocolate Mixture. Add The Flour Mixture In 2 Batches, Whisking To Blend Completely Each Time.
Pour The Batter Into The Prepared Pan. Bake 22 To 25 Minutes Until A Skewer Inserted 1 Inch From The Edge Comes Out Clean.
When Inserted In The Center, A Bit Of Moist Batter Will Cling To It. Do Not Over Bake. Cool The Cake On A Rack 10 Minutes, Then Invert Onto A Plate.
Invert Back Onto The Rack So The Shiny Side Is Up. Cool The Cake Completely Before Sliding It Onto A Serving Plate.
Sift The Cocoa Over The Top.