Instructions for 1000 watt microwave: Lightly grease a standard rimmed baking sheet or cover with parchment paper. You can also use a silicone baking mat. (Use nonstick cooking spray)
In a large microwave safe bowl stir together the sugar, corn syrup and salt. Microwave on high for 4 minutes until the mixture is bubbly and the sugar has dissolved. Stop and stir halfway through cooking. (At the 2 minute point.)
After cooking for 4 minutes total, add the peanuts. Stir until fully coated.
Return the bowl to the microwave and continue cooking for an additional 3 minutes stopping to stir halfway through cooking. (At 1 1/2 minutes.)
At the end of cooking the peanuts in the syrup for 3 minutes, stop and add the cubed butter. Stir well, then return to the microwave, cooking for 1 minute longer.
Carefully remove the bowl from the microwave. Add the vanilla and baking soda stirring constantly. The mixture will foam up, continue to stir until it dies down, slightly.
Quickly pour the mixture onto the prepared pan and use an offset icing spatula sprayed with cooking spray, to spread to approximately 1/4-inch thickness.
Allow the peanut brittle to cool completely, then break into desired size pieces.
Store in an airtight container at room temperature for up to 2 weeks. (Makes about 1 1/2 lbs)