Cream butter and sugar together. Stir in egg yolks, adding one at a time.
Sift together flour, cocoa and salt and add to butter. Mix until dough just comes together. Shape into a disc and chill until ready to roll. If preparing ahead of time, pull dough from fridge an hour before rolling.
Preheat oven to 350 F.
On a lightly floured surface, roll out dough to just over 1/4-inch thick. Cut dough using cutters of your choice. Lift cookies gently onto a parchment-lined baking sheet carefully (they’re fragile) and bake for 12-15 minutes. Allow to cool.
Melt chocolate with water, directly on low heat, stirring until smooth and shiny. Remove from heat and stir in butter and sour cream until melted.
Stir in egg yolks, one at a time, mixing gently and slowly.
Whip whites with salt to soft peaks, adding sugar once whites become foamy. Fold into chocolate mixture in thirds.
Pour into a serving dish.
Chill mousse for at least 3 hours before serving.