Preheat the oven to 350 F. Grease an 8-inch springform pan and line the bottom with parchment paper.
Whisk the flour, sugar, cocoa powder, baking soda, and salt together. Add the egg, oat milk, oil, and vanilla, then whisk in the hot coffee until the thin batter is smooth.
Pour the batter into the pan and bake for 24 to 28 minutes, until the center springs back when lightly pressed. Cool the cake completely in the pan.
Sprinkle the gelatin over the cold water and let it bloom for 5 minutes. Meanwhile, place the dairy-free chocolate in a heat-safe bowl.
Heat the coconut cream until steaming, then stir in the bloomed gelatin until dissolved. Pour the mixture over the chocolate, wait 2 minutes, and stir until glossy and smooth.
Whip the dairy-free whipping cream, powdered sugar, vanilla, and salt to medium peaks. Once the chocolate mixture is barely warm, fold in one-third of the whipped cream, followed by the remainder.
Spread the mousse evenly over the cooled cake. Refrigerate for at least 6 hours, then release the springform pan and serve the cake chilled.