Preheat oven to 350 degrees F. Measure flour by lightly spooning the flour into the measuring cup, not by scooping the measuring cup into the flour. If you scoop the cup into the flour, you'll have too much flour and the cake will be dry.
In a large bowl, combine butter, sugar and brown sugar, and beat until mixture is light and fluffy. Add melted and cooled chocolate, beating until combined.
Add eggs, one at a time, beating thoroughly after each addition until eggs combine with butter mixture (this can take some time). Gradually add powdered sugar, mixing well. By hand, stir in flour, cocoa and pecans until well blended.
Spray a 12-cup Bundt pan with nonstick baking spray. Spoon batter into prepared pan. Bake at 350 degrees F for 46 to 52 minutes, until crust looks dry and shiny (traditional doneness tests won't work because of the soft fudge center). Cool cake in pan on rack for 1 hour, then invert onto serving plate and remove pan. Cool completely. Sprinkle with powdered sugar before serving.