Once the cake has cooled, it's time to prepare it for the mousse topping. Place the springform ring around the cake but do not secure it yet. Place parchment paper inside the ring between the outer ring and the cake. Secure the ring to tightly wrap the parchment around the cake like a collar. Set aside.
Create a double boiler by filling a medium sauce pan with 1 inch of water and setting over medium heat. Place a large heat-proof bowl on top of the saucepan, making sure it does not touch the water. Add chopped chocolate and stir continuously to help the chocolate melt slowly. Once melted, remove from the saucepan and on top of a towel. Turn off heat and pour out water. Wipe the inside of the saucepan clean.
Add gelatin and water to a small bowl and mix together. Let sit for 3 minutes while the gelatin blooms. Meanwhile, place ¾ cup of heavy cream in the clean sauce pan and set over medium low heat. We want it to be hot but not boiling. Once the cream has reached this point, add bloomed gelatin and whisk to melt the gelatin into the cream. Remove from heat.
Add 3 egg yolks to the melted chocolate and whisk together quickly. It will firm up fast. Add half of the hot cream to the chocolate and whisk together to loosen it up. Once combined, add the rest of the cream and whisk until smooth. Set aside.
In the clean bowl of a stand mixer. add the egg whites and cream of tartar. Using the whisk attachment, whisk on medium until the eggs start to foam. Add the ¼ cup of sugar and continue on medium speed until the whites have turned to fluffy meringue that looks like shaving cream. Pour meringue into a medium bowl and clean the whisk and bowl. Add 1 cup of the whipped cream to the bowl of the stand mixer and whisk on medium to turn to whipped cream.
Gently fold the whipped cream into the chocolate mixture until it's all one uniform color. Do the same with half of the whipped meringue. And then again with the other half, being careful to keep air in the mousse.
Pour the mousse over the cooled cake to create a flat, even layer. Place in the fridge to set up for at least 4 hours, but preferably overnight.