Line a 24 x 28 cm baking sheet with parchment paper.
Separate the eggs. Beat egg white with a pinch of salt and lemon juice until firm. Chill in the refrigerator until ready to use.
Put the margarine and sugar in a bowl and beat with the whisk on medium speed until the sugar stops crunching. The mass should have become noticeably lighter. Add the egg yolks by tablespoon, then the vanilla extract. Beat for 4 minutes.
Mix the two flours with the baking powder, cocoa, grated walnuts and a pinch of salt. Stir into the dough alternately with the addition of drinking yoghurt.
Preheat the oven to 170 degrees bottom heat.
Finally fold in the egg white. Pour the batter into the baking sheet and use a spatula to spread it evenly. Bake on medium heat for 50 - 60 minutes.
Let cool overnight in a baking sheet. The next day, wash and peel the mangoes and fillet them into approx. 5 mm thick slices. Cover the cake with it, add the walnut halves and protect the fillets with the couverture. Let ripen in the refrigerator for at least 5 hours.