Combine sugar, brown sugar, butter, corn syrup and salt in 1 1/2 - 2 quart microwave-safe bowl. Microwave on high power until boiling (about 1 minute). Continue to microwave on high power for 2 minutes after boiling point is reached. Remove from microwave and stir in baking soda and vanilla. Place popped popcorn (and nuts, if desired) in large microwave-safe bowl. Pour hot syrup over popcorn. Microwave on high for 1 1/2 minutes. Stir well. Microwave on high for an additional 1 1/2 minutes (1 minute). Stir again. If corn is very sticky, microwave another 30-90 seconds (1 minute). Pour into shallow pan and allow to cool, stirring occasionally to keep from "clumping."