Grease a baking sheet heavily with unsalted butter and set aside.
In a large microwavable bowl, combine the sugar and corn syrup, and microwave on HIGH for 4 minutes.
200 g granulated sugar, 120 ml golden corn syrup
Stir in peanuts and return to microwave on HIGH for 3½ to 4 minutes
220 g peanuts
Quickly stir in the butter, baking soda and vanilla until mixture is light and foamy.
15 g unsalted butter, 1 tsp baking soda, 1 tsp pure vanilla extract
Spread as thinly as possible onto the prepared baking sheet. Be careful the mixture is very hot. Use a lightly buttered offset spatula to help with the spreading.
Let cool for 1 hour before breaking into pieces. Store in an airtight container.