CI November: Triple Chocolate Mousse Cake

I got invited over to a colleague's house for Thanksgiving dinner.  I volunteered to bring a dessert, and since the pies were already covered I decided to make the triple chocolate mousse cake from Cook's Illustrated.  I've been wanting to make this for a while but never had a good excuse.  It's a bit too rich and fancy for an impulse bake, and there's no way I'd trust myself with it with no one to help me eat it.
Fortunately, I'm not the only one blogging the recipes from CI, so I don't have to type everything in, although I've changed the commentary as needed.  I've pilfered this one from the aptly named Annie's Eats.

Ingredients:
For the bottom layer:
6 tbsp. unsalted butter, cut into 6 pieces (plus extra for greasing the pan)
7 oz. bittersweet chocolate, finely chopped
¾ tsp. instant espresso powder
1½ tsp. vanilla extract
4 large eggs, separated
Pinch of salt
1/3 cup light brown sugar, packed

For the middle layer:
2 tbsp. cocoa powder, preferably Dutch-processed
5 tbsp. hot water
7 oz. bittersweet chocolate, finely chopped
1½ cups heavy cream
1 tbsp. granulated sugar
Pinch of salt

For the top layer:
¾ tsp. powdered gelatin
1 tbsp. water
6 oz. white chocolate, finely chopped
1½ cups heavy cream
For garnish:
Chocolate curls or cocoa powder
Directions:
To make the bottom layer, butter the bottom and sides of a 9-inch springform pan.  Your springform pan should be at least 3" tall.   Center a rack in the oven and preheat to 325˚ F.  Combine the butter, chocolate, and espresso powder in a large heatproof bowl and microwave carefully until chocolate is almost melted.  Stir occasionally until the mixture is smooth.  Remove from the heat and let cool slightly, about 5 minutes.  Whisk in the vanilla and egg yolks.  Set aside.
In a stand mixer fitted with the whisk attachment, beat the egg whites and salt on medium speed until foamy, about 30 seconds.  Crumble half of the brown sugar into the mixing bowl with your fingers to remove any lumps.  Beat until incorporated, about 15 seconds.  Add the remaining brown sugar and continue to beat on high speed until stiff peaks form, about 1 minute more.  Using a whisk, mix one-third of the beaten egg whites to the chocolate mixture to lighten it.  Fold in the remaining egg whites gently with a rubber spatula until no streaks remain.  Pour the batter into the prepared springform pan and smooth the top with a spatula.
Bake until the cake has risen, is firm around the edges and the center has just set but is still soft (should spring back after pressing gently with a finger), about 14-18 minutes.  Transfer the cake to a wire rack to cool completely, at least 1 hour.  Do not remove the cake from the pan.  (If not making the second layer right away, cover with plastic wrap and refrigerate until ready to proceed.)

To make the second layer, whisk together the cocoa powder and hot water in a small bowl; set aside.  Melt the chocolate in a heatproof bowl set over simmering water, stirring occasionally until smooth, or microwave it carefully Once melted, remove from the heat and let cool slightly, 2-5 minutes.
In the clean bowl of a stand mixer fitted with the whisk attachment, whip the cream, sugar and salt on medium speed until the mixture begins to thicken, about 30 seconds.  Increase the speed to high and whip until soft peaks form when the whisk is lifted, 30-60 seconds.  Whisk in the cocoa powder mixture until smooth.  Using a whisk, mix one-third of the whipped cream to the chocolate mixture to lighten it.  Fold in the remaining whipped cream gently with a rubber spatula until no streaks remain.  Pour the mousse into the springform pan over the cooled cake and tap gently on the counter 3 times to remove air bubbles.  Gently smooth the top with a spatula.  Wipe the inside edge of the pan to remove any drips.  Refrigerate for at least 15 minutes while preparing the top layer.  I covered it with plastic wrap and put it in the fridge overnight.
To make the top layer, sprinkle the gelatin over the water in a small bowl and let stand at least 5 minutes to soften.  Place the white chocolate in a medium bowl.  Bring ½ cup of the cream to a boil in a small saucepan.  Remove the pan from the heat, add the gelatin mixture and stir until dissolved.  Pour the hot cream mixture over the white chocolate and let stand about 1 minute.  Whisk until the mixture is smooth.  Cool to room temperature, about 5-8 minutes, stirring occasionally.

In the clean bowl of a stand mixer fitted with the whisk attachment, whip the remaining 1 cup of cream at medium speed until it begins to thicken.  Increase the speed to high and whip until soft peaks form when the whisk is lifted, 30-60 seconds.  Using a whisk, mix one-third of the whipped cream to the white chocolate mixture to lighten it.  Fold in the remaining whipped cream gently with a rubber spatula until no streaks remain.  Spoon the white chocolate mousse into the pan over the middle layer.  Smooth the top with an offset spatula.  Return the cake to the refrigerator and chill until set, at least 2½ hours.  (Note: Do NOT cover your pan with plastic wrap unless you are completely sure you can do so without it touching the top of your cake during placement or removal.  If it does touch the cake, it will smudge the perfectly pristine top, forcing you to make an excessive amount of chocolate curls.  I speak from experience.)

OK, old biddy Annie commentary again...The base layer took longer than expected. I left it in there around 20 minutes.  I thought it was done in the middle but it started looking gooier as it sunk and cooled.  It was somewhat gooey but ok in the final cake.  The second and third layers were pretty straightforward.  I was out of chocolate so I didn't put any chocolate shavings on top, but it would've looked nice.  Some of the commenters suggested using a collar of greased parchment paper around the cake to make look prettier when it is removed from the pan.  It did stick a bit, but no one seemed to mind since they were fighting to lick the knife.
It was really rich, but since it was fluffy and not too sweet it was not overwhelming.  I'll definitely make it again when I need a showstopper dessert.