Separate the egg yolks from the whites and whisk the latter until stiff after adding the cream of tartar.
Separately, whisk the egg yolks with sugar for at least 10 minutes: the mixture should be white and fluffy.
Always beaten, slowly add the vanilla first, then the salt and the melted but warmed butter, then the sifted flour and cocoa.
Finally add the milk.
Finally, gently incorporate the egg whites that have been previously whipped into a snow with a movement from the bottom to the top.
There will be a rather liquid dough.
Pour the mixture gently into a pan lined with parchment paper and bake in a preheated oven at 150 ° C for about 80 minutes, or until the surface is golden brown.
Let the cake cool completely, then cover it with plastic wrap and put it in the fridge for at least 2 hours.
Finally cut the magic cake into cubes, sprinkle with icing sugar and serve.
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