• Preheat the oven to 350 degrees. Grease the inside of a 9-inch springform pan. You can use up to a 10.5-inch springform pan but don't use anything smaller than 9 inches. Remove the ring and place a piece of parchment paper over the bottom liner. Secure the ring again over the paper. There will be excess paper hanging out the sides. This will help the cake from leaking through the cracks in the ring.
• In a medium bowl, add flour, cocoa powder, cornstarch, baking soda, baking powder, and salt. Whisk together to combine. If your mixture has lumps from the cocoa powder, sift through a sieve. Set aside. In the bowl of a stand mixer, add sugars and canola oil. Mix on low with a paddle attachment to combine until the mixture resembles wet sand. Add eggs one at a time. Then add vanilla extract and sour cream. Mix together until combined.
• Add 1/3 of dry ingredients to the mixture while on low speed, followed by half of the coffee. Repeat with another 1/3 of the dry ingredients and rest of the coffee. Finish with the last third of the dry ingredients. The mixture will be very wet.
• Pour the batter into the prepared springform pan and into the oven for 25-30 minutes, or until the center is set and a toothpick comes out clean after being inserted. Remove from the oven and let cool for 10 minutes. Remove the ring and clean the edges to remove the crumbs. Let the cake cool completely before making the mousse.

For the mousse:

• 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.

For the ganache:

• One the mousse is set, remove the cake from the fridge. Remove the collar and ring and place over a cooling rack atop a baking sheet. Set aside. Heat the ¾ cup of heavy cream in the microwave until hot and foaming around the edges, about 1-2 minutes. Place 6 ounces of the chopped chocolate into a measuring cup with a spout. Pour hot cream over the chocolate. Do not stir together yet.
• Let the cream and chocolate sit together untouched for 6 minutes. After that time has elapsed, whisk together to create the silky smooth ganache. Pour ganache into the center of the mousse layer, allowing it to slowly make it's way to the edge and eventually topple over the edge. Quickly before the chocolate sets, us an offset spatula to pour more over the edge, if needed.
• Give the cake about 30 minutes to set before slicing into it. Additionally, refrigerate for up to 24 hours before serving.