Instructions: Place rack in center of oven; preheat oven to 300 degrees. Line an 8- by 8- by 2-inch pan with aluminum foil, shiny-side up. Spread room-temperature butter all over the bottom and sides. Pour bread crumbs into pan; tilt pan to coat with crumbs. Shake out excess.

Sift together flour, baking powder and salt; set aside.

In a small bowl, beat eggs, vanilla and sour cream just to mix; set aside.

In a medium saucepan, melt 1/2 cup butter over moderate heat. Add sugar; stir until partially melted. Add chocolate; stir until melted. Transfer to a large mixing bowl. Stir in egg mixture, then dry ingredients. If the mixture is not perfectly smooth, beat it briefly with an electric mixer or a hand-held beater. Stir in nuts. Pour into baking pan; smooth top.

Bake for about 1 hour, until a toothpick gently inserted in the middle comes out almost clean. Let stand at room temperature until completely cool.

Invert onto small cookie sheet or board. Remove pan and foil. Cover with a piece of waxed paper; invert onto another small cookie sheet or board. Brownies will be right side up. Chill, then cut into squares.

Yields 16 brownies.

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