Bring whipping cream to a simmer in a medium saucepan. Remove from heat; cool 5 minutes.
Process chocolate in a food processor 15 to 20 seconds or until finely chopped. With processor running, slowly add cream; process until smooth. Add 1/3 cup sugar and butter. Add egg yolks, 1 at a time; process until smooth. Add vanilla, and process until blended.
Combine breadcrumbs and chocolate mixture, stirring until blended.
Beat egg whites at medium speed with an electric mixer until soft peaks form. Gradually add remaining 1/3 cup sugar, and beat until mixture is glossy and stiff peaks form.
Fold one-third of egg white mixture into chocolate mixture. Fold in remaining egg white mixture until blended. Spoon into a greased 8-inch square pan. Place pan in a broiler pan; add enough water to broiler pan to reach halfway up sides of square pan.
Bake, on center oven rack, at 325° for 45 minutes or until a knife inserted in center comes out clean. Cool on a wire rack 10 minutes. Carefully invert onto a serving platter. Serve with Custard Sauce.
Note: For testing purposes only, we used Ghirardelli Semi-Sweet Chocolate.