1. Preheat the oven to 180°C (350°F).
2. Whisk the whole eggs with the sugar, then add the melted butter and sifted flour (with baking powder + salt).
3. Divide the dough into 2 portions in 2 different bowls (2/3 and 1/3).
4. In the smaller portion, add the 2 tablespoons of cocoa powder.
5. Add the vanilla extract to the other bowl.
6. Butter and flour 2 rectangular molds and fill each one with one of the preparations and bake for 15 minutes in the oven (or one at a time if you only have one mold).
7. Unmold onto a rack and let cool.
8. For assembly, cut the 2 cakes into equal rectangles (level the height if necessary). Cover one vanilla rectangle and one chocolate rectangle with chocolate sauce.
9. Place the chocolate cake on top of the vanilla cake covered with chocolate sauce, and the plain vanilla cake on top.
10. Make a glaze by whisking the egg white with powdered sugar.
11. Glaze the Napolitain cake with the white glaze and sprinkle with chocolate vermicelli.
12. Chill for 1 hour before serving.