Line two cavities of your cupcake pan with liners or grease two 4oz. ramekins. Set aside.
Preheat oven to 325 degrees F.
In a medium mixing bowl, whisk together the dry ingredients ( flour, sugar, cocoa powder, soda, baking powder and salt ). Set aside.
In a small measuring cup, stir together water, oil, vanilla and vinegar. Add to dry ingredients and stir.
Divide into two liners. Bake 18 to 20 minutes, or until toothpick inserted in the center of each cupcake comes out clean. Cool completely.
In a small bowl, mix together cocoa powder and hot water. Set aside to cool slightly.
In a double boiler, melt chocolate. Cool 3 to 5 minutes.
In a large mixing bowl of a stand mixer, whip cream, sugar and salt at medium speed. Whip for 30 seconds. It should slightly thicken. Increase the speed to high and whip the cream until soft peaks form. It should happen quite fast ( 20 to 30 seconds ) so do not walk away. Set aside.
Whisk cocoa powder mixture with melted chocolate until smooth. At first it will look like your chocolate seized but keep whisking. It will come back together and be smooth.
With whisk, fold in ⅓ of the whipped cream into chocolate mixture to lighten it. Switch to a rubber spatula and gently fold in the rest of the whipped cream, until no streaks remain. It is important or the mousse won't set right.
Place cream in a medium mixing bowl. Add sugar and salt and whip for 30 seconds. It will slightly thicken. Increase the speed to high and whip until stiff peaks form, about 3 to 4 minutes. Set aside.
Crumble the cooled cupcakes. Spoon the crumbs into the bottom of your serving glasses.
Drizzle with rum, one teaspoon per glass.
Spoon the mousse, dividing equally among the glasses.
Top with whipped cream and smooth out with the back of a spoon.