Add the butter into a large microwave-safe bowl. Add peanut butter to the bowl, covering the butter. This will prevent the butter from popping and overheating while it cooks.
Microwave at full power for 30 seconds. Remove and stir with a whisk. If butter is not completely melted, microwave for 30 more seconds and whisk again.
Add the powdered erythritol to the peanut butter mixture and stir until batter is smooth.
Place batter back into microwave and cook for 1 minute. Remove and whisk until smooth. This will help the sugar melt into the peanut butter mixture. Microwave for 1 more minute and whisk until smooth.
Spread batter into the prepared loaf pan. Freeze for about 1 hour or until fudge is cooled and set.
Let fudge soften for a few minutes and then cut with a sharp knife into 1 inch squares. You should have enough for 18 pieces.
Notes
For photo purposes, I did cut the fudge pieces a little bigger but this fudge is very rich so I usually cut into 1 inch wide squares (the fudge is about 3/4 inch thick).
Even if the peanut butter mixture looks smooth after whisking in the erythritol, you still want to heat it up for those extra 2 minutes (at 1 minute intervals) to help dissolve the erythritol.
Make sure to use powdered erythritol because granular will not melt into the fudge.
This fudge can also be prepared on the stove in a double boiler. Just make sure to heat long enough after the sweetener is added to allow it to melt into the peanut butter mixture.
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The estimated nutrition carbohydrates count excludes sugar alcohols. The net carbs excludes both fiber and sugar alcohols.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.