Heat the oven to 180C/fan 160C/Gas 4. Butter a solid-based 22cm round cake tin.
Put the caster sugar and 2 tbsp water in a small saucepan over a low heat to allow the sugar to dissolve. Bring to the boil and continue to cook without stirring until the sugar has turned a deep caramel colour. Add 40g butter and swirl the pan to incorporate evenly. Pour immediately into the cake tin, covering the base with an even layer of caramel. Leave to cool.
Peel the mangoes, cut off the cheeks and slice into wedges. Slice the rest of the mango flesh and arrange all the mango over the hard caramel in the cake tin.
Cream the softened butter and the caster sugar until pale and fluffy. Gradually add the beaten eggs, mixing well between each addition. Sieve the flour, baking powder and a pinch of salt together and fold into the cake mixture using a large metal spoon. Add the milk and mix until smooth.
Carefully spoon the cake mixture over the mango slices and level. Bake for about 45-50 minutes or until a skewer inserted into the middle of the cake comes out clean. Allow the cake to cool in the tin for 5 minutes before turning out onto a serving dish.