Combine butter, corn syrup, sugar and salt in an 8 cup bowl. Microwave on high, 1-2 minutes or until it bubbles. Boil for 3 minutes, stirring after each minute. Add baking soda and vanilla, stir well, pour over popcorn.
Shake well, fold-down bag and microwave on high for 1 minute. Shake well. Microwave on high for 1 minute. Shake well. Microwave on high for 30 seconds. Shake well. Microwave on high for 30 seconds.
Place on wax paper and let cool. Store in an airtight container!