Line the sides and base of a 13 by 9 by 2-inch baking pan with foil or parchment paper.
Melt the butter and dark chocolate together in a large heavy based pan over a low heat. In a bowl or large measuring glass, beat the eggs together with the sugar and vanilla extract. Allow the chocolate mixture to cool a little, then add the egg and sugar mixture and beat well. Fold in the flour and salt. Then stir in the white chocolate. Beat just to combine and then scrape into the prepared pan. Bake for about 25 minutes. You can see when the brownies are ready because the top dries to a slightly paler brown speckle, while the middle remains dark, dense and gooey. Even with such a big batch you do need to keep checking on it; the difference between gooey brownies and dry ones is only a few minutes. Remember, too, that they will continue to cook as they cool.
To serve, cut into squares while still warm and pile up on a large plate, sprinkling with powdered sugar pushed with a teaspoon through a small sieve and dotting with birthday candles as you go. Set alight and enjoy the festive scene.