These brownies are rich and fudgy. The presence of sour cream makes them especially moist, with a rich chocolate flavor.
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INGREDIENTS
1/4 cup (31 grams) sifted unbleached all-purpose flour
1/4 teaspoon baking powder
1/4 teaspoon salt
8 ounces (225 grams) bittersweet or semi-sweet chocolate; if have large chunks, chop into 1/2-inch size pieces
5 ounces (140 grams) (1 1/4 cups) walnuts, chop into medium size pieces
2 large eggs
2 teaspoons (8 grams) vanilla extract
1/3 cup (80 grams) sour cream
4 ounces (1 stick) (113 grams) unsalted butter
3/4 cup (150 grams) sugar

INSTRUCTIONS
Preheat oven to 350 degrees F and position rack in center of oven. Prepare an 8- x 8- x 2- inch pan by lining the pan with a piece of aluminum foil. Coat pan with room temperature butter or cooking oil spray. 

Sift together the flour, baking powder, and salt; set aside. In a In a small bowl beat the eggs, vanilla, and sour cream just to mix; set aside.

In a 1 quart saucepan, over medium heat, melt the butter. Add the sugar and stir for a minute until sugar is partially melted. Then add the chocolate and stir until melted.

Transfer to large mixing bowl. Stir in egg mixture, then the sifted dry ingredients. Don't worry if the mixture is not perfectly smooth. Stir in nuts.

Pour into the baking pan and smooth top. Bake for about 50 – 55 minutes, until a toothpick inserted in the middle comes out almost (but not completely) clean. Let stand at room temperature until cool. Remove from pan by inverting on a cutting board and then re-inverting it onto a plate. Chill brownies completely before cutting.

STORAGE
Store brownies at room temperature. Brownies freeze very well.

Recipe adapted from Maida Heatter's Cookies