Spread the popcorn and peanuts on a baking sheet and keep warm in your oven set to 300 degrees F while you prepare the butter toffee. You don't need to preheat the oven.
2. Melt the butter in a medium saucepan over medium/low heat.
3. Add the sugar and corn syrup and simmer, stirring occasionally. Pop a candy thermometer into the mixture and watch it closely.
4. When the thermometer reaches 300 degrees F pour the candy over the warm popcorn and peanuts. Stir well so that the candy coats the popcorn. Put the popcorn back into the oven for 5 minutes, then stir it again to coat the popcorn. Repeat if necessary to thoroughly coat all of the popcorn.
5. Pour the coated popcorn and peanuts onto wax paper or a silicone pad. When cool, break up the chunks into bite-size pieces and store it all in a covered container