- Prepare equipment: If using silicone bar molds, set them on a tray for stability. Alternatively, line a small loaf pan with parchment (for slicing later). Chill molds lightly if desired.
- Whip the cream: In a large chilled bowl, beat the cold heavy cream with an electric mixer on medium speed until it begins to thicken. Add the powdered sugar, sifted cocoa powder, and vanilla extract (and a pinch of espresso powder if using). Continue whipping to soft peaks — the mousse should be light, airy, and hold its shape but still be easy to pipe or spoon.
- Fill molds: Spoon or pipe the mousse into the silicone molds, filling each cavity and smoothing the tops. If using a loaf pan, spread mousse evenly to about 1–1½ inches thick. Tap the molds lightly on the counter to remove large air pockets.
- Freeze solid: Transfer the tray or pan to the freezer and freeze for at least 4–6 hours, or preferably overnight, until the mousse is completely firm.
- Melt the coating chocolate: When the mousse is fully frozen, prepare the coating. In a microwave-safe bowl, melt the chopped chocolate (and coconut oil, if using) in 20–30 second bursts, stirring between intervals until smooth and glossy. Alternatively, use a double boiler to melt gently.
- Unmold and dip: Remove frozen bars from molds. Working quickly to minimize thawing, dip each frozen mousse bar into the melted chocolate, allowing excess coating to drip off. Place dipped bars on a parchment-lined tray. If desired, sprinkle toppings (nuts, sea salt, freeze-dried fruit) immediately before the coating sets.
- Set and store: Return the dipped bars to the freezer until the chocolate is firm, about 15–30 minutes. Store bars in an airtight container layered with parchment between layers, kept in the freezer until ready to serve.
- Serve: Remove bars from the freezer about 2–3 minutes before serving so they are easier to bite but still fully frozen. Enjoy straight from the freezer.
