Fudge Brownie Muffins

4 ounces semisweet chocolate, coarsely chopped
8 tablespoons unsalted butter
1 1/2 cups all-purpose flour, sifted
1/4 cup unsweetened non-alkalized cocoa powder, sifted
2 teaspoons double-acting baking powder
1 teaspoon baking soda
1/8 teaspoon salt
2 large eggs, lightly beaten (at room temperature)
2/3 cup granulated sugar
1/2 cup milk
1 1/2 cups semisweet chocolate chips
1 cup coarsely chopped nuts

Position a rack in the center of the oven and preheat to 350