Coat a 12-cup muffin pan with cooking spray and put a cookie in the bottom of each cup. Whisk the eggs, sugar, salt and water in a large heatproof bowl. Bring a few inches of water to a simmer in a medium saucepan and set the bowl on top (do not let the bowl touch the water). Cook, whisking, until pale, fluffy and very thick (like a thick pudding), 25 to 40 minutes, depending on the size of your bowl. Remove from the heat and whisk in the melted chocolate; let cool.
Beat 1 1/4 cups heavy cream with the sour cream in a large bowl with a mixer on medium-high speed until stiff peaks form. Gently fold into the cooled chocolate mixture until no white streaks remain, then divide evenly among the muffin cups. Cover and refrigerate until set, 12 hours or overnight.
Run an offset spatula around each mousse cup to loosen and slide the spatula under the cookie; remove the mousse cups. (If they’re too soft to remove, freeze briefly.) Beat the remaining 1/2 cup heavy cream in a large bowl with a mixer until soft peaks form, then dollop onto the mousse cups and sprinkle with shaved chocolate.