Pre heat oven to 160C degrees and toast the nuts for approx. 6-10 minutes, if using whole walnuts chop into smaller pieces. Line a 20cm square tin with baking paper.
In a medium size saucepan, combine peanut butter, choc chips, maple syrup, salt and cinnamon. Warm the pot on a low-medium heat, stirring continuously, until the mixture is melted and combined. Remove from heat.
Stir in the vanilla extract, followed by the oat flour, coconut and then the walnuts. The mixture should have dried a little and it should be tough to mix the walnuts through.
Put the mixture into the tin and spread using the back of a spoon. Sprinkle the salt flakes (if using) and place some baking paper over the top, using a flat bottomed drinking glass press down and smooth out the mixture.
Cover and refrigerate for 2-3 hours (you can also freeze for 30-45 minutes if you require a quicker result.)