Line a baking tray with baking paper and set it aside. Roughly cut the butter into cubes. Place the butter in a large microwave-safe bowl and melt on high for 40 seconds. If the butter still has solid lumps, return to the microwave for a further 20-30 seconds. Remove from the microwave. Add the powdered erythritol to the butter. Use a whisk to combine to a smooth paste.
Microwave the butter and sweetener for 2 minutes on high. It will be runny and very hot. Remove from the microwave and take care when handling the bowl. Add 3 tablespoons of peanut butter. Whisk to combine. Microwave again for 2 more minutes.
Add 1 teaspoon of vanilla extract and 1 cup of peanuts. If you have whole peanuts, it is recommended to roughly chop the peanuts into smaller pieces. Stir to coat all the peanuts in the mixture.
Pour the peanut mixture into the prepared baking tray. Use a spatula or the back of a spoon to level out the mixture. Be sure to work quickly here as the erythritol will begin to harden.
Either leave the tray at room temperature or place it in the fridge to harden faster. Break the brittle into equal-sized chunks and enjoy! 1 serving is 1oz/ 30g.