Preheat oven to 350 degrees. Place eight 6-ounce (3 1/2-inch diameter) ramekins on a rimmed baking sheet, and coat with cooking spray; set aside.
Stir flour, cocoa powder, sugar, baking soda, baking powder, and salt in the bowl of an electric mixer. Attach bowl to mixer fitted with the paddle attachment. Add egg, milk, oil, vanilla, and 1/4 cup water; mix on medium-low speed until smooth and combined, about 3 minutes.
Divide batter evenly among prepared ramekins. Bake until a cake tester inserted into the centers comes out clean, about 20 minutes. Transfer to a wire rack; let cool completely. Run a knife around sides of cakes; unmold. Cakes can be refrigerated, wrapped in plastic, up to 1 day.
Trim each cake to 1 inch high. Transfer to a baking sheet lined with parchment paper. Cut eight 10 3/4-by-4-inch strips of parchment paper. Wrap a parchment collar around base of each cake, keeping bottom flush with baking sheet. Secure each collar with tape; set aside.
Transfer bittersweet-chocolate mousse to a large pastry bag fitted with a large round tip (such as Ateco #808). Pipe a 1-inch layer of mousse into each parchment collar. Refrigerate until mousse is set, about 20 minutes. Repeat with milk chocolate mousse, piping on top of the bittersweet chocolate mousse. Refrigerate at least 4 hours and up to overnight.
Microwave semisweet chocolate until slightly warm but not melted, about 30 seconds. Scrape at a 45-degree angle with a vegetable peeler, forming curls. Before serving cakes, remove parchment collars, and garnish with chocolate curls.