MOUSSE Melt chocolate chips in double boiler or microwave (careful not to burn!), then cool. Whisk egg whites with salt until foamy, then add sugar. Beat on high until very firm. Whip heavy cream until very firm. While whisking egg yolks, gradually add chocolate, then whisk in whipped cream until completely smooth. Fold in egg whites. Refrigerate a few hours. CUPCAKES Preheat oven to 350°F. Line tin. In small bowl, mix boiling water and cocoa until smooth. Add sour cream and vanilla. In separate bowl, whisk flour, baking powder, baking soda and salt. In a large bowl, cream butter and sugar until light and fluffy. Add eggs one at a time. Scrape down bowl. Mix in a third of flour mixture, half cocoa mixture, a third flour, remaining cocoa, and remaining flour. Fill cups 2/3 full. Bake 18 minutes until toothpick comes out clean. GANACHE Bring heavy cream to simmer. Add chips and stir until completely smooth. ASSEMBLY Cut a hole into top of each cupcake. Use bag to pipe mousse into hole. Cover with ganache.