Heat oven to 325 degrees. Lightly butter and flour a 9x9x2-inch square cake pan. Sift flour, cocoa, baking powder and salt together onto a sheet of waxed paper. In top of a double boiler melt chocolate and butter over medium-high heat, stirring occasionally until smooth. Remove from heat.
In an electric mixer mix eggs, sugar and vanilla on high until slightly thickened, about 1 1/2 minutes. Add melted chocolate mixture and mix on medium for 30 seconds. Add flour mixture and mix on low 10 seconds, then on medium 10 seconds. Add sour cream and mix on medium 5 seconds.
Scrape around sides and bottom of bowl with a rubber spatula to combine thoroughly. Use spatula to fold in chopped chocolate. Pour batter into cake pan and spread evenly. Bake until toothpick inserted in center comes out clean, about 40 minutes. Allow brownies to cool 10 to 15 minutes before cutting with a serrated knife.