- Adjust oven rack to lower-middle position and heat oven to 325 degrees. Butter sides of 9-inch springform pan; flour sides and tap out excess. Line the bottom with parchment paper and butter and flour the top of it also.
- Wrap bottom and sides of pan with 2 sheets of HEAVY-DUTY aluminum foil. Fold both edges down and away from pan.
- Melt 12 tblsp butter and chocolates in large bowl over large saucepan containing about 2 quarts barely simmering water, stirring occasionally, until chocolate mixture is smooth. Cool mixture slightly, then whisk in vanilla and egg yolks. Set chocolate mixture aside, reserving hot water, covered, in saucepan. Keep the water on very low burner to keep it warm.
- In clean bowl of standing mixer fitted with whisk attachment, beat egg whites and salt at medium speed until frothy, about 30 seconds; add half of crumbled brown sugar, beat at high speed until combined, about 30 seconds, then add remaining brown sugar and continue to beat at high speed until soft peaks form when whisk is lifted, about 2 minutes longer. Using whisk, stir about one-third of beaten egg whites into chocolate mixture to lighten it, then fold in remaining egg whites in 2 additions using whisk. Gently scrape batter into prepared springform pan, set springform pan in large roasting pan, then pour hot water from saucepan to depth of 1 inch. Carefully slide roasting pan into oven; bake until cake has risen, is firm around edges, center has just set, and instant-read thermometer inserted into center registers about 170 degrees, 45 to 55 minutes. (Keep the thermometer in hot water and test quickly.) It will be a bit wiggly.
- Remove springform pan from water bath, recycle foil, and cool on wire rack 10 minutes. Run thin-bladed knife between sides of pan and cake to loosen; cool cake in springform pan on wire rack until barely warm, about 3 hours. Take off the springform outer ring, then wrap cake (with bottom) in plastic wrap and freeze.
- Get 12 pieces of plastic wrap ready. Turn the cake upside down on its plastic wrap and take off the bottom of the pan and the parchment paper. Then slice the cake (still upside down) into 12 pieces, wrap them up, and store in a ziploc freezer bag in the freezer.
- Instant dessert!
Source: Mom via Cooks Illustrated