Mousse:
1 teaspoon unflavored gelatin
2 tablespoons water
6 ounces white chocolate, broken into pieces
1 tablespoon instant espresso powder
1/4 cup boiling milk
1 tablespoon Cognac
1 1/2 teaspoons vanilla extract
1 cup chilled whipping cream
4 English toffee candy bars (about 5 ounces total), chopped into
pea-size pieces

Glaze:
6 ounces semisweet chocolate, broken into pieces
3 ounces unsweetened chocolate, broken into pieces
1/2 cup confectioner's sugar
1/2 cup plus 1 tablespoon unsalted butter, melted and sizzling hot
1 1/2 teaspoons Cognac
1/2 cup whipping cream, whipped
Chocolate coffee bean candies

For cake: Position rack in center of oven and preheat to 325 degrees F. Butter 12-by-8-inch pan. Line bottom with parchment; butter paper. In work bowl of food processor fitted with steel knife, coarsely chop chocolate with sugar using about 6 on/off turns, then process continuously until chocolate is as fine as sugar. With machine running, pour boiling water through feed tube and process until mixture is smooth, stopping once to scrape down sides of work bowl. Add eggs and cocoa and mix 1 minute. Add butter pieces and process 1 minute. Add sour cream and Cognac and blend 5 seconds. Spoon flour, baking powder, baking soda and salt in circle atop mixture and blend using 3 on/off turns. Run spatula around inside of work bowl. Blend just until ingredients are incorporated, using 2 to 3 on/off turns; do not overprocess. Spread batter in prepared pan. Bake until tested inserted in center comes out clean, about 15 minutes. Cool completely in pan. Refrigerate until well chilled. (Can be prepared 2 days ahead and refrigerated or 2 months ahead and frozen. Wrap tightly.)
For mousse: Sprinkle gelatin over water in small heatproof bowl. Let stand until gelatin softens, about 2 minutes. Place bowl in pan of simmering water and stir until gelatin dissolves completely. (Mixture can also be dissolved in microwave. Cook on medium for approximately 50 seconds.) In work bow of food processor fitted with steel knife, coarsely chop white chocolate with espresso powder using 6 on/off turns, then process continuously until chocolate is ground. With machine running, pour boiling milk through feed tube and process 2 minutes. Let mixture rest 30 seconds. Process until completely smooth, about 30 seconds. Add gelatin mixture, Cognac and vanilla and mix 5 seconds. Transfer to large bowl. Cool completely. Using electric mixer, beat 1 cup cream to soft peaks. Fold whipped cream and toffee into white chocolate mixture. Chill at least 2 hours or overnight.
To assemble: Discard paper from cake. Trim edges of cake to even. Cut into four 4-by-10-inch strips. Arrange 1 cake strip in center of plate. Slide waxed paper strips under cake to keep plate clean. Gently spread cake with 1/3 of mousse. Top with second cake layer and spread with mousse. Repeat with remaining layers and mousse, ending with cake layer. Gently press on sides to neaten. Cover and refrigerate until set, about 30 minutes.
For glaze: Insert steel knife in food processor. Coarsely chop both chocolates with confectioner's sugar using about 6 on/off turns, then process continuously, until chocolate is ground. With machine running, pour hot butter through feed tube and process until chocolate is melted and mixture is smooth, stopping once to scrape down sides of work bowl. Add Cognac and blend 3 seconds. Cool slightly. Spread very thin layer of glaze over top and sides of cake. Refrigerate cake until glaze is set, about 15 minutes. Set aside 1/3 cup glaze for decoration. Rewarm remaining glaze until spreadable; spread over top and sides of cake. Spoon reserved 1/3 cup glaze into pastry bag fitted with small star tip. Pipe decorative border around top edge of cake. Chill at least 2 hours. (Can be prepared 3 days ahead. Wrap tightly.) Let stand at room temperature 1 hour before continuing. Spoon whipped cream into pastry bag fitted with star tip. Pipe cream in lattice pattern atop cake. Place coffee bean candies at cross-points.