Preheat oven to 400 degrees. Line the cups of a muffin pan with paper liners. In medium bowl, sift together flour, cocoa, baking soda, baking powder, and salt. In a large bowl with an electric mixer, beat together butter and sugar on medium speed until creamy. Add eggs and vanilla and beat until light and fluffy. Stir in the sour cream and 3/4 cup chocolate chips. Add the flour mixture, stirring just until incorporated. Spoon the mixture into the muffin cups, filling each cup about three quarters full. Sprinkle the tops of the muffins with the remaining 1/4 cup chocolate chip. Bake for 20 to 25 min, until firm to the touch. Remove the pan from the oven and let stand for 1 or 2 min, and then transfer the muffins in their paper cups to a wire rack.
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