Preheat the oven to 350 degrees. Line an 8 inch square pan with aluminum foil, letting the foil overhang two opposing sides of the pan by about 2 inches. Grease bottom of foil only.

Combine the butter and chopped chocolate in a microwave-safe bowl. Microwave on high for 30 minutes and stir. Continue microwaving at 50% power and stirring every 30 seconds until chocolate is melted. Let cool to warm.

In another bowl, thoroughly stir together the flour, cocoa powder and salt; set aside.

In a mixing bowl, combine the sugar and brown sugar with the chocolate-butter mixture. Add the eggs and vanilla and stir until the sugar dissolves and the mixture is well blended and smooth. Stir in the flour mixture just until evenly incorporated.

Put batter in pan and spread evenly spreading to the edges. Bake in the middle of the oven for 28-30 minutes, or until the center is almost firm when tapped and a toothpick inserted in the center comes out clean except for the bottom ¼ inch, which should still look moist.

Transfer the pan to a wire rack and let stand until the brownie is completely cooled. Refrigerate until well chilled.

Lift from pan by grasping foil. Cut into squares.