Preheat oven to 400 degrees. Combine sweet potatoes, butter, egg yolks, brown sugar, salt, ginger, cinnamon, nutmeg and evaporated milk. Mix well. Beat egg whites until stiff peaks form, add sugar and fold into sweet potato mixture. Pour into pie shell and bake for 10 minutes. Reduce heat to 350 degrees and bake for 30 minutes, or until firm.
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