Line a large baking sheet with parchment paper or a silicone baking mat and set nearby. Have a heatproof spatula or wooden spoon and oven mitts ready.

In a large microwave-safe bowl, combine 1 cup granulated sugar and 1/2 cup light corn syrup; stir until evenly mixed.

Microwave the sugar mixture uncovered on high for 1 minute, then stir. Continue microwaving in 30–60 second intervals, stirring after each interval, until the mixture is bubbling vigorously and is a deep amber color (about 4–6 minutes total depending on your microwave). Use caution—the syrup will be extremely hot.

Immediately stir in 1 cup salted roasted peanuts. Return the bowl to the microwave and cook on high in 30–60 second intervals for about 1–2 minutes more, stirring once, until the mixture is bubbling and the peanuts are hot.

Remove the bowl from the microwave (use oven mitts). Quickly add 1 Tbsp butter, 1 tsp vanilla extract, and 1 tsp baking soda, and stir vigorously. The mixture will foam and lighten in color.

Working quickly while the mixture is still hot and foamy, pour it onto the prepared baking sheet and spread to your desired thickness with the spatula.

Let the brittle cool completely at room temperature until hard (about 20–30 minutes).

Break the cooled brittle into pieces and store in an airtight container.