16 tablespoons unsalted butter, softened
2 cups granulated sugar
8 ounces bittersweet chocolate, melted and slightly cooled
2 teaspoons vanilla extract
4 eggs
1 cup unbleached all-purpose flour
1 teaspoon salt
1 cup chopped walnuts
2 cups semisweet chocolate chips
2 cups milk chocolate chips
2 cups butterscotch chips

Preheat oven to 350 degrees F. Line an 8 3/4 x 13 1/2-inch glass or metal pan with a large piece of aluminum foil, smoothing it into the corners. Butter the foil.

In a large bowl, cream butter with the sugar until it is light in color. Stir in melted chocolate. Ad vanilla extract, then add the eggs, beating until blended.

In another bowl, combine flour and salt. Add walnuts and all the chips, tossing to coat them with the flour. Scrape the flour, nuts and chips into the bowl with the butter, and stir with a wooden spoon just until all the ingredients are mixed. Do not over-beat. Scrape the batter into the prepared pan, holding the foil so that it does not move. Place the pan in the middle of the oven and bake until a wooden pick inserted in the center comes out almost clean, about 50 minutes.

Remove the pan and let it stand for one hour, then cover the pan with foil and place it in the freezer for at least 1 1/2 hours or in the refrigerator overnight.

Turn the cooled brownies out onto a clean workspace, lift off the foil and cut into 32 squares. Store in the refrigerator or the freezer in an airtight tin.