Preheat the oven to 350°F (175°C) and line a 9" springform pan with parchment paper.

In a large mixing bowl, whisk together the cake flour, baking powder, baking soda, salt, and granulated sugar.

Add the milk, neutral oil, flax egg, and vanilla extract, stirring until the batter is smooth.

Pour the mixture into the prepared pan and spread it evenly.

Bake for approximately 15 minutes until the cake is lightly browned and a toothpick comes out clean. Let it cool completely on a wire rack.

Bloom the gelatin powder in cold water for about 5 minutes.

Blend the mango chunks until smooth to create a mango puree.

Gently heat the bloomed gelatin until it dissolves completely, then mix into the mango puree.

Whip the cream with powdered sugar until soft peaks form and fold into the mango mixture.

Pour the mango mousse over the cooled cake and smooth the top. Chill in the refrigerator for at least 2 hours.

Heat the reserved mango puree with sugar over medium heat until it simmers.

Mix cornstarch with water to create a slurry and add to the simmering mango mixture, stirring continuously.

Cook until the curd thickens, then stir in butter.

Allow to cool before pouring over the set mousse layer.

Chill the cake for 12 hours or overnight before serving.