Coat 6 x 3-1/4 x 2-1/8-inch loaf pan with nonstick cooking spray. Trim a sheet of waxed paper to line bottom and long sides of pan, slightly overhanging edges.
Sprinkle gelatin over water in small saucepan; let stand 5 minutes to soften. Place chopped white baking bars in medium-size bowl.
Add butter and 1/3 cup heavy cream to softened gelatin. Heat over medium-high heat until butter melts and gelatin is dissolved. Pour over chopped white bars. Let stand 2 minutes. Stir until smooth.
Place bowl in an ice-water bath; stir in whipped cream. Stir mixture until it thickens slightly.
Tilt prepared pan sideways 45 degrees. (Use foil to prop it up in a larger pan.) Pour slightly thickened white mixture into tilted pan, smoothing top even. Refrigerate or freeze while preparing chocolate layer.
Place chocolate in small bowl. Heat 1/2 cup cream and butter in small saucepan over medium-high heat until cream just begins to bubble. Pour over chocolate in bowl; let stand 3 minutes. Stir until smooth. Fold in whipped cream. Place in ice bath; stir until slightly thickened.
Remove loaf pan from fridge. Gently touch white layer: It should be firm. Rest pan flat on counter; pour in chocolate mixture, smoothing top. Chill 30 minutes or until firm.
Run a knife along ends to loosen dessert. Use waxed paper to remove loaf from pan. Use a warm knife to cut loaf into 6 slices. Garnish with berries and mint, if desired.