Place brownies in the freezer for 45 minutes or until firm for cleaner slices.
Combine cocoa and sugar. Add melted butter and next 4 ingredients, stirring just until combined. Pour mixture into a lightly greased 8-inch square pan.
Bake at 350° for 25 to 30 minutes. Cool. Cut into 2-inch squares, and dust with powdered sugar before serving.
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