Melt chocolate and butter together, either in the microwave (at 30 second intevals, stirring well between each. It should take a minute or so to melt), or on top of a double boiler without letting the bottom of the pan touch the water. Transfer chocolate mix to a bow and let cool a few minutes.
Add sugar and mix well with a spatula. Add eggs, egg yolk, vanilla and mix until smooth. Don´t beat the batter.
Finally add the flour, cocoa, baking powder and salt, sifting it directly on top of the chocolate batter. (You can sift all dry ingredients before onto a piece of wax paper or a bowl too, and then just add them without sifting again). Mix well with the spatula.
Spread evenly on top of the blondie layer. To do this, drop the thick batter in tablespoons all through the surface, then spread carefully with the back of a spoon or a spatula. The warm blondie layer will help the spreading a bit.
Bake for about 25 minutes, until it´s dry on top and a tester comes out with moist (but not wet) crumbs attached. Don´t let it dry or the bars will be tough.
Let cool on a wire rack. Unmold, drizzle with dulce de leche or caramel and serve.