1. Preheat oven to 350°F (175°C). Grease a 9-inch springform pan and line sides with parchment paper, extending about 1 inch above the rim.
2. In a medium bowl, whisk together flour, cornstarch, cocoa powder, baking soda, baking powder, and salt.
3. In a large bowl, whisk sugars and oil until like wet sand. Add eggs one at a time with vanilla extract, whisking thoroughly. Stir in sour cream.
4. Add half the dry ingredients, then hot coffee, then remaining dry ingredients, mixing until smooth.
5. Pour batter into prepared pan; bake 25-30 minutes. Cool completely, then level cake top.
6. Mix hot coffee and cocoa powder for the soak, then spoon over cooled cake.
7. Bloom gelatin in 3 tablespoons water. Melt chocolates together, then cool slightly.
8. Stir egg yolks into chocolate. Heat 3/4 cup cream, dissolve gelatin, and mix with chocolate.
9. Whip 1 cup cream to stiff peaks. Beat egg whites with cream of tartar to soft peaks, adding sugar gradually.
10. Fold whipped cream into chocolate mix, then fold in meringue until uniform.
11. Pour mousse over cake in pan, chill 4+ hours.
12. For ganache, heat cream and pour over chocolate, whisking till smooth. Pour over chilled mousse cake.
13. Chill at least 30 minutes before serving, dust with cocoa or garnish if desired.