Make the fudge: In a medium-sized, heavy-bottomed pot, combine the nut butter, chocolate chips, sweetener, butter, salt and cinnamon. Warm the pot over medium heat, stirring often, until the mixture is melted throughout. Remove the pot from heat.
Toast the nuts: Arrange the nuts in a single layer on a small, rimmed baking sheet (I used parchment paper for easy clean-up). Bake for 7 to 10 minutes, until fragrant (7 minutes for thinner/smaller/chopped nuts and about 10 for whole pecans). If you're using large nuts like pecans, transfer them to a cutting board and chop them into small pieces with a chef's knife.
Prep work: Preheat oven to 325 degrees Fahrenheit. Cut two strips of parchment paper to fit across the interior of an 8 to 9-inch square baker. Criss-cross the papers at the bottom of the baker and fold the ends up the sides of the baker (see photos). If you need to make your own oat flour, blend 1 and ¾ cups oats in a blender or food processor until ground into a fine flour.