Preheat oven to 350°F (175°C). Grease a 6-inch springform pan and line the bottom with parchment paper.
In a bowl, mix oat milk, orange juice, orange zest, vanilla extract, vegetable oil, and brown sugar.
In another bowl, whisk together flour, cocoa powder, baking powder, baking soda, and salt.
Combine wet and dry ingredients until smooth. Pour batter into prepared pan and bake for 25–30 minutes.
Cool the cake completely in the pan before removing.
Melt vegan chocolate and let it cool slightly.
In a clean bowl, whip aquafaba with cream of tartar until stiff peaks form. Gradually add powdered sugar.
Gently fold in melted chocolate to create mousse. Pour over the cake base and smooth the top.
Refrigerate for at least 4 hours or overnight until set.