Line a square 8-inch pan (or a similar capacity rectangular pan) with wax paper.
In a medium saucepan over low heat, add the peanut butter, butter, vanilla, salt, and stevia.
Whisk with a fork until smooth.
Remove from the 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 the fudge into 25 squares and serve.
Video
Notes
Right after making it and cutting it into squares, this fudge is a bit soft. It improves the next day after an overnight rest in the fridge, so it's a good idea to make it the day before.
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.
The amount of stevia glycerite I use equals about 3 tablespoons of sugar. Make sure to use a powdered sweetener to keep the texture smooth.
You can keep this fudge in an airtight container in the fridge for up to a week, or in the freezer for up to three months. Serve this fudge straight from the fridge or freezer. It softens quickly.