1. Preheat the oven to 350 degrees F.
2. Line a 9 x 13 inch baking pan with foil and grease the foil lightly.
3. In a microwave safe bowl, add and combine butter with the unsweetened and semi sweet chocolate and microwave it at medium power for 2 minutes or until the chocolate has melted.
4. In a bowl, add and combine the all purpose flour, unsweetened cocoa powder and salt. Mix well and set it aside.
5. In an electric mixing bowl, add and beat the eggs.
6. Stir in the granulated sugar and brown sugar and beat this on high speed for 10 minutes until it doubles in volume.
7. Using a spatula gently fold in the chocolate-butter mixture.
8. Gently fold in the dry ingredients until well combined.
9. Take the prepared baking pan and pour the above batter evenly into the pan. Bake this in the preheated oven at 350 degrees for 20 - 32 minutes or until a toothpick inserted comes out clean.