Line two baking sheets with parchment paper or nonstick foil.

Use a 6-quart slow cooker. Place the butter into the slow cooker insert and add the sugar and corn syrup on top. Pour in the vanilla.

Cover and cook on high for 1 hour. Stir well, then add the baking soda. (It will sizzle and the mixture will be milky-yellow and the sugar might still look granulated.)

Cover again and cook on high for another 30 minutes. Stir in the peanuts. Cover and cook for another 15-30 minutes, or until the sugar mixture is brown and looks the color of peanut brittle.

Using oven mitts, carefully pour half of the pot onto each baking sheet and spread out with a spatula. Let cool at room temperature for about an hour, then break into pieces.