This exceedingly moist and rich (but so simple!) cake is all about chocolate and butter; if you can swing it, choose top-quality brands.
Preheat oven to 350°. Grease an 8-inch springform pan with the softened butter. Dust with the cocoa powder; tap out excess.
Break up chocolate into a medium microwave-safe bowl. Add butter chunks. Zap in the microwave 1 minute. Stir with a flexible spatula. If need be, zap another 30 seconds to melt. Stir until smooth.
Crack eggs into the bowl of a stand mixer fitted with a whisk attachment. Whip on high speed, gradually cascading in sugar and salt, until pale, thick, and tripled in volume, about 3 minutes. (Lacking a stand mixer, use a handheld mixer and coordination, or a balloon whisk and upper-arm strength.)
Pour chocolate mixture into egg mixture; beat on low to combine. Sprinkle in flour; beat on low just to combine. Mix in any renegade specks of flour with a flexible spatula. Transfer batter to prepared pan.
Bake until puffed and surface is dry and a toothpick jabbed in center only just comes out clean. (Begin checking at 35 minutes.) Don't fret over any cracks in the top. Cool on a wire rack 20 minutes; remove sides of pan. Enjoy warm or at room temp with whipped cream.
For perfect sweet whipped cream, follow a Rule of One: Combine 1 cup heavy cream, 1 tablespoon granulated sugar and 1 teaspoon vanilla. With an electric or stand mixer, whip on medium speed to soft peaks. Cover and chill.