1-Preheat and prepare pan: Preheat oven to 350°F, spray an 8-inch springform pan with nonstick spray, and line bottom with parchment paper.

2-Make brownie batter: Melt unsweetened chocolate and butter together in a saucepan over medium-low heat, stirring constantly; remove from heat, whisk in sugar, cool slightly, then whisk in eggs and vanilla until smooth.

3-Bake brownie base: Sift flour, cocoa powder, and salt into the bowl, fold just until combined, spread batter evenly in pan, and bake 20-25 minutes until a toothpick comes out with moist crumbs; cool completely on a wire rack.

4-Bloom gelatin: Sprinkle gelatin over 1 tablespoon water in a small bowl and let sit 5 minutes until spongy.

5-Make chocolate base for mousse: Heat ½ cup heavy cream and 2 tablespoons sugar until sugar dissolves and cream is hot (not boiling); remove from heat, stir in bloomed gelatin until dissolved, then pour over 6 ounces chopped semisweet chocolate; let sit 1 minute, whisk until smooth, and cool to room temperature.

6-Whip cream for mousse: Whip remaining 1 cup cold heavy cream to medium-stiff peaks.

7-Fold mousse: Fold one-third of whipped cream into cooled chocolate mixture, then fold in remaining whipped cream until no streaks remain.

8-Assemble mousse layer: Spread mousse evenly over cooled brownie base and refrigerate while preparing ganache.

9-Heat cream for ganache: Heat ¾ cup heavy cream and 1 tablespoon butter over medium-high heat until just beginning to boil; remove from heat.

10-Make ganache: Pour hot cream over 9 ounces semisweet chocolate in a bowl; let sit 1 minute, stir until smooth and shiny, then cool until not warm to the touch.

11-Top with ganache and chill: Pour ganache over chilled mousse, spread gently, and refrigerate at least 1 hour until firm.

12-Serve: Remove springform sides, peel away parchment, transfer cake to platter; garnish if desired; slice cold and let slices sit at room temperature 30 minutes before serving.