Preheat the oven to 170 °C and line a large baking sheet with parchment paper.

Sift the gluten-free flour, cocoa powder, baking powder, and a pinch of salt into a mixing bowl.

In another bowl, whisk together the oat milk and the lemon juice or vinegar. Let sit for 5 minutes so it curdles slightly.

Add the liquid mixture to the dry ingredients and whisk until smooth.

Pour the batter onto the baking sheet and spread it into a thin, even layer about 1/2 cm thick.

Bake for 13–16 minutes, or until the sponge releases easily from the parchment. Let it cool completely before cutting.

Whisk together all the ingredients until the cocoa powder dissolves. Set aside or refrigerate until you're ready to assemble the cake.

If you don't have Boiron mango purée, blend fresh or frozen ripe mango until smooth, then strain if needed to remove the fibers.

In a small saucepan, whisk together the mango purée, agar-agar powder, whipping cream, and lemon juice. Bring quickly to a boil, whisking constantly. Cover and keep warm to prevent the mixture from setting.

Melt the dark chocolate in a heatproof glass bowl over a double boiler or in the microwave.

Set a bowl over a saucepan of hot water. Add the mango-agar mixture and the melted chocolate, then whisk or blend with an immersion blender until smooth. Keep the bowl over the hot water to keep the mango-chocolate mixture warm. Agar-agar sets quickly at room temperature, so it's important to keep the mousse warm until you're ready to assemble the cake.

Whip the cream until softly whipped. Fold half into the warm mango-chocolate mixture until smooth, then fold in the rest until fully incorporated. Set the mousse over the saucepan of hot water to keep it warm.

Prepare the cake ring and acetate: line an 18 cm metal cake ring with a 17 cm-high strip of acetate and place it on a large plate or cake board. Make sure the acetate sits snugly against the inside of the ring for clean, smooth sides.

Cut the sponge to fit the cake ring. Place the first chocolate sponge layer in the bottom of the ring, making sure it fits snugly and is evenly centered. Brush it with the mango-chocolate syrup.

Spread a layer of mango-chocolate mousse over the sponge. Use an offset spatula to spread it evenly to about 1.3 cm thick. Gently tap to release any air pockets.

Continue alternating layers of chocolate sponge and mango-chocolate mousse. Keep each mousse layer roughly the same thickness for an even, neat finish.

You should end up with eight layers: four layers of chocolate sponge and four layers of mango-chocolate mousse.

Make the top mousse layer slightly thicker. Use the tip of the spatula to swirl from the center outward, creating a spiral pattern on top.

Refrigerate for at least 4–6 hours or overnight, until the mousse is firmly set. For a quicker set, freeze the cake for 1 hour.

Once the cake is firm enough, dust the top with cocoa powder. Carefully remove the cake ring, then gently peel away the acetate to reveal the sides.

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