Prepare all your ingredients, have them measured out and ready to go. Prepare a non stick baking mat or a lightly buttered baking sheet.
Stir together karo syrup and sugar in a microwave safe bowl.* Place in microwave for 4 to 5 minutes, or until the sugar has dissolved and the surface of the mixture is covered in clear rapid bubbles.
Remove carefully from microwave*. Stir in peanuts quickly and return to microwave and heat for another 3 to 5 minutes or until sugar and peanut mixture has turned a pale butterscotch color. Watch carefully, pausing the microwave to check if necessary to avoid burning of your sugar.
Remove from microwave carefully, and stir in the vanilla and butter. The mixture will sizzle and deepen in color.
When completely mixed in, return to microwave and heat another 1 to 2 minutes. Use an instant read thermometer to check the temperature, when it has reached 300°F, remove from microwave.
Quickly add and stir in the baking soda. It will cause the mixture to foam rapidly. Stir the soda in completely then pour on your prepared cooling surface. Try to spread the mixture evenly as you pour.*
Let cool for 30 minutes (up to an hour). When completely cool and hard, break into shards and store in an airtight container. Layer between wax paper and store in a dark, cool place for longest lasting quality. Safe to store up to 2 months.
*Using a clear bowl is recommended. Use a bowl no smaller than 1.5 qts. *USE POTHOLDERS! The bowl will be hot and only get hotter, do not handle without hand protection. *Once poured, you can also quickly spread it with your spatula to even it out. Do this quickly to prevent collapsing the air from the baking soda.