Preheat your oven to 175°C (350°F). Beat the eggs and sugar until well combined, then slowly add the flour and baking powder. Pour the batter into a greased cake pan and bake for about 20 minutes, or until a toothpick inserted comes out clean. Let it cool.
In a saucepan, heat the milk and heavy cream until warm. In a separate bowl, whisk the egg yolks, flour, and sugar. Gradually add the warm milk mixture to the egg mixture, stirring constantly. Return the mixture to the saucepan and cook over low heat, stirring until thickened. Remove from heat and stir in the vanilla extract. Let it cool.
Whip the Cream: In a bowl, whip the heavy cream with powdered sugar until soft peaks form.
Assemble the Tart: Slice the sponge cake into layers. Spread a layer of vanilla custard on the bottom layer, then a layer of whipped cream. Repeat with the remaining layers, finishing with whipped cream on top.
Decorate the top with pieces of the sponge cake to resemble mimosa flowers.