Preheat oven to 250°F. Put popped popcorn (and nuts if desired) in a nonstick or buttered roasting pan or metal bowl; place in oven.
Microwave: Melt butter in 2-quart glass bowl in microwave (High 1-1½ minutes). Stir brown sugar and corn syrup into butter. Microwave High for 5 minutes -stopping the cooking to stir every 2 minutes. The mixture should be very bubbly (temperature 250-255°F). Note: For 650-750 Watt ovens, an additional minute or two of cooking time may be necessary; stir after each additional minute of cooking time. Stir baking soda into hot mixture.
Stove: Melt butter in heavy 2-quart sauce pan over medium heat; stir in sugar and corn syrup; bring mixture to a boil and then allow to boil 5 minutes without stirring.) Stir baking soda into hot mixture.
Drizzle over warmed popped popcorn and toss using 2 wooden spoons or heat resistant rubber spatulas to coat popcorn.
Bake coated popcorn for 45 minutes, stirring every 15 minutes. Remove from oven and allow to cool. Store in an airtight container.