1. In a bowl, melt half of the butter with milk chocolate and stir until smooth (I used microwave).
2. Add half of the sugar and whisk until smooth. Whisk in half of the eggs. Add half of the salt and half of the flour and mix until well combined.
3. In another bowl, melt the rest of the butter with white chocolate and stir until smooth.
4. Add the rest of the sugar and whisk until smooth. Whisk in the rest of the eggs, salt and flour until well combined.
5. Grease an 8-inch baking dish. Preheat oven to 350 degrees F (180 degrees C).
6. Spoon the batter into the dish, alternating milk and white chocolate batter.
7. Using a skewer (or a knife) make some swirls.
8. Bake for 30 minutes, until the middle is set.
9. Let to cool completely and cut into squares.