Put the poppy seeds in a bowl and pour 300 ml of boiling water over them and let them steep for about 15 minutes. Pour the poppy seeds into a sieve, drain thoroughly and chop in a universal blender (this will make the poppy seeds a little more homogeneous).
Coarsely chop the hazelnuts. Separate eggs. Cream egg yolks and 275 g sugar for 3-4 minutes. Mix in poppy seeds, hazelnuts, almonds, starch and lemon juice. Beat egg white and salt in a large bowl until stiff and fold into the poppy seed mixture in 2-3 portions.
Pour the dough into a greased springform pan (26 cm Ø) and bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes. Remove and let cool for at least approx. 2 hours.
Mix the sauce powder with 1 tablespoon of sugar and stir with at least 6 tablespoons of 250 ml milk until smooth. Bring the remaining milk to the boil, remove from the heat and stir in the sauce powder with a whisk. Let sauce boil for about 1 minute while stirring.
Remove the sauce from the stove and let it cool down for about 15 minutes.
Cut the mango from the stone and peel the flesh. Cut 1/2 half of the mango into small cubes, dice the remaining mango roughly. Put vanilla sauce and coarse mango cubes into a high mixing cup and puree.
Whip the cream until stiff and spread loosely on the cake with a spoon. Spread the mango vanilla sauce over it and add the mango cubes.