Line a square 8-inch baking dish or a similar-capacity food storage container with wax paper, leaving an overhang.
Place the chocolate pieces, butter cubes, peanut butter, stevia, and vanilla in a medium saucepan. Heat over very low heat, stirring often until melted. Be careful not to burn the chocolate.
Remove the mixture from the heat as soon as the chocolate is melted. Keep stirring until smooth.
Using a rubber spatula, transfer the mixture to the prepared pan, spreading it out evenly and smoothing the top. Cover with plastic wrap. Refrigerate the fudge until set, for about 4 hours.
Uncover and use the wax paper overhang to remove the fudge from the pan and onto a cutting board. With a sharp knife, cut it into 16-20 squares.
Serve immediately, or refrigerate for later.
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Notes
I use natural creamy peanut butter. The only ingredients are peanuts and salt. If your peanut butter doesn't contain salt, add a pinch of salt to the mixture.
I highly recommend using a fresh jar when making this recipe. The peanut butter at the bottom of these jars is often quite dry, even if you stirred it thoroughly when you first opened it.
The fudge is still a bit soft after the initial four hours. Its texture improves after an overnight stay in the fridge.
The fudge softens quickly at room temperature, so it's best enjoyed straight out of the fridge.
After the fudge has set and you've cut it into squares, you can store it in an airtight container in the fridge for up to a week. Keto fudge can also be frozen. Arrange the squares in a single layer in a freezer bag and freeze them for up to three months.