In a medium saucepan over low heat, add the peanut butter, butter, vanilla, salt and stevia.
Whisk with a fork until smooth.
Remove from heat and whisk in the coconut flour.
Using a rubber spatula, pour the mixture into the prepared pan.
Cover and place in the freezer until set, at least 1 hour and preferably 90 minutes.
Cut into 25 squares and serve. Keep leftovers in the fridge or in the freezer, in an airtight container.
Right after making it and cutting it into squares, this fudge is a bit soft. It actually improves the next day, after an overnight rest in the fridge. So it's not a bad idea to make it the day before you plan on enjoying it.
It's best to use a freshly opened peanut butter jar when making this recipe. I find that the peanut butter at the bottom of these jars is often too dry, even if you've stirred it thoroughly.