Line a loaf pan or square pan with parchment paper. Place 3/4 cup of the peanuts in an even layer on the bottom and set aside.
In a saucepan and on low heat, combine the butter and sugar substitute. Once the butter has melted and sugar has dissolved, hold the candy thermometer into the center of the saucepan. Stir the mixture continuously, until the thermometer reaches 300F (hard crack) and remove from the heat. Continue stirring, to ensure the butter/sugar mixture doesn't separate.
Pour the toffee over the peanut butter and let sit for 5 minutes. After 5 minutes, top remaining 1/4 cup of peanuts over the top. Refrigerate until firm. Once firm, break apart into pieces.
Storing peanut butterHomemade peanut brittle should generally not be stored at room temperature. At room temperature, the toffee becomes soft and can be difficult to hold shape. Peanut brittle should be stored in the refrigerator at all times, in a sealed container. Refrigerated peanut brittle will last at least 4 weeks. Freezing peanut brittleYou can easily freeze peanut brittle to enjoy later. Either store the peanut brittle pre-cutting or in individual pieces. If you store the peanut brittle in the freezer without breaking into pieces, wrap it completely in parchment paper and place it in a ziplock bag. To enjoy the brittle from the freezer, you need to thaw overnight in the fridge.If you freeze individual pieces, you need to wrap them in parchment paper too and place them in individual ziplock bags.Frozen peanut brittle will keep 6 months.