Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan with oil. In a bowl, whisk together flour, cocoa powder, sugar, and baking soda. Add oil, vinegar, and water, mixing until smooth.

Pour batter into the springform pan and bake for 20 minutes, until a toothpick comes out clean. Cool completely in the pan on a wire rack.

While cooling, whip coconut cream or aquafaba until light and airy, about 2-3 minutes. Fold in cocoa powder and sugar gently to keep airiness.

Once cool, remove the cake from the pan and layer half of the mousse over it. Chill in the refrigerator for about 30 minutes.

Melt dark chocolate and coconut cream over low heat until smooth. Let it cool slightly, then pour over the set mousse layer.

Chill the cake for at least 3 hours. Slice with a warm knife and serve chilled.