Preheat the oven to 338°F (170°C) and line a wide baking sheet with parchment paper.

In a mixing bowl, sieve together the dry ingredients: gluten-free flour, cocoa powder, baking powder, and a pinch of salt.

In another bowl, combine oat milk and apple cider vinegar. Let this mixture sit for 5 minutes to curdle slightly.

Combine the wet mixture with the dry ingredients, whisking until smooth.

Pour the batter evenly onto the baking sheet and spread it into a thin, even layer (about half a centimeter thick).

Combine mango puree, agar-agar powder, heavy cream, and lemon juice in a small saucepan. Whisk continuously while bringing it to a quick boil. Cover and keep in a warm place to prevent it from curdling.

Melt dark chocolate in a Pyrex bowl over a double boiler or microwave.

In a bowl placed on top of a pan with hot water, combine the mango-agar mixture with the melted chocolate using a whisk or hand blender until smooth. Keep the bowl on the hot water pan to warm the mango-chocolate mixture. Agar-agar sets quickly at room temperature, so it’s important to keep the mousse warm for the cake assembly later.

Whip the heavy cream until slightly stiff. Fold half of the whipped cream into the warm mango-chocolate mixture until homogenous. Fold the remaining whipped cream until the mousse is smooth and well-mixed. Place the mousse on the pan filled with hot water to keep it warm.

Prepare cake ring and acetate: Line an 18 cm metal cake ring with a 17 cm tall acetate cake sheet on top of a large plate of cake board. Ensure the acetate is snug against the inner wall of the cake ring for clean, smooth sides.

Add a layer of mango chocolate mousse on top of the sponge. Use an offset spatula to spread it evenly, about 1.3 cm (1/2 inch) thick. Gently tap to smooth out any air pockets.

Continue alternating between layers of chocolate sponge and mango chocolate mousse. Each mousse layer should be approximately the same thickness, keeping the height of each layer consistent.

By the end, you should have eight layers: four chocolate sponge layers and four mango chocolate mousse layers.

The topmost mousse layer should be thicker. Use the spatula’s tip to swirl from the center to the edges, creating a spiral effect on top.

Refrigerate for at least 4–6 hours or overnight to allow the mousse to set firmly. You can also freeze the cake for an hour to set quickly.

When the cake is firm enough, sprinkle the top with cocoa powder. Carefully remove the cake ring. Gently peel away the acetate sheet to reveal the sides of the cake.

The finished cake should be approximately 16 cm tall, with alternating layers visible from the sides. Slice using a warm knife and serve.