Line a baking sheet with parchment and lightly butter it. Set nearby with a heatproof spatula.
Combine sugar, corn syrup, and salt in a large microwave-safe glass bowl. Stir until the sugar looks moistened.
Microwave on High for about 3 minutes 30 seconds, until the syrup is bubbling vigorously.
Stir in the peanuts to coat. Microwave again for 2 to 3 minutes, until lightly amber and fragrant.
Add butter and vanilla carefully; stir until melted. Microwave 45 to 75 seconds, until deep golden and very fluid.
Sprinkle in baking soda and stir briskly. The mixture will foam and lighten in color.
Immediately pour onto the prepared pan. Tilt and gently spread thin with the spatula; work quickly.
Cool completely until hard, about 30 minutes. Break into pieces to serve.