Bottom Layer:

1. Spray an 8 or 9 inch springform pan generously with cooking spray. Adjust your oven rack to the middle position and preheat the oven to 325 degrees F. Melt the butter and chocolate together in a microwave bowl on half power, stirring every minute until chocolate is smooth. When smooth, let it cool for about 5 minutes. Then whisk in the vanilla and egg yolks and set aside.
2. In a stand mixer, beat the egg whites and salt with the whisk attachment at medium speed until lots of bubbles appear about 30 seconds (frothy). Add half the brown sugar and mix for another 15 seconds or so. Then add the remaining brown sugar and switch the speed to high, beating until soft peaks form when whisk is lifted, about a minute to minute and a half. Scrape down the bowl with a spatula half way through if needed. Watch carefully so as to not over-whip to stiff peaks.
3. Using a whisk, gently mix one-third of beaten egg whites into the chocolate mixture to lighten it. Switching to a rubber spatula, and FOLD (not mix) in the remaining whipped egg whites until no white streaks remain. Gently poor the mixture into the springform pan and smooth the top with an offset spatula.
4. Bake for 13 to 18 minutes, when the cake has risen, is firm around the edges and the center springs back after a gentle touch of the finger. Remove from the oven and cool on a wire rack for about an hour. The cake will collapse as it cools and pull away from the edge of the pan. Wash your stand mixer and whisk attachment for the next layer.

Middle Layer:

5. Mix the cocoa powder into the hot water and set aside. Melt the chopped chocolate again in a microwave bowl and stir occasionally until completely melted and smooth. Remove from heat and set aside to cool. While waiting, whip cream, granulated sugar and salt in your stand mixer with the whisk attachment at medium until frothy. Move speed to high and whip until soft peaks form, about a minute. This goes fast so watch carefully!

6. Next, whisk the cocoa powder-water mixture into the melted chocolate until smooth and shiny. Then using a hand whisk, fold 1/3 of the whipped cream into the chocolate mixture to lighten. With a rubber spatula, gently fold in the remaining cream until no white streaks remain. Gently pour the mousse into the springform pan over the bottom layer and tap the bottom on the counter a few times to remove any air bubbles. Smooth the top and refrigerate at least 15 mins while preparing the top layer. Wash your stand mixer equipment again for the final layer.

Top Layer:

7. In a small container, sprinkle the gelatin over the water. Place white chocolate into a medium sized mixing bowl. Bring 1/2 cup heavy cream to simmer in a small saucepan over medium-high heat, stirring constantly. Remove from heat and mix in the gelatin mixture. Stir until fully dissolved.

8. Pour the cream mixture over the white chocolate and stir until melted and smooth. Cool to room temperature, mixing occasionally.

9. Whip remaining cream in the bowl of your stand mixer at medium speed until frothy, and then increase the speed to high and whip until soft peaks form when whisk is lifted, about one minute. Using a hand whisk, fold 1/3 whipped cream into white chocolate mixture until thoroughly combined. Then using a rubber spatula, gently fold in the remaining whipped cream until fully incorporated. (It’s a little hard to tell since you’re mixing white into white essentially, just do your best to watch it and not fold to much or it will deflate.) Gently pour out onto the middle layer and smooth the top. Refrigerate until set, approximately 2 and 1/2 hours.

To remove and serve: Run a knife between the cake and the side of the pan. Unbuckle the side of the pan and remove. Run a clean hot knife along the outside of the cake to smooth the sides. Use a clean hot knife or wire to slice and serve. Garnish with chocolate curls or strawberries if desired.