Preheat oven to 350F / 180C
Prepare your muffin pan by either greasing or lining, as required
In a wide bowl, add peanut butter, milk, cinnamon or vanilla, salt, baking powder, and sugar
Mix well until all ingredients are combined
Add the flour and fold in without mixing too much
Pour into muffin pan until each hole is 3/4 full
Bake in pre-heated oven for 20-25 minutes or a skewer inserted into the muffins comes out clean
Cool on a rack and store in an airtight container
Keeps at room temperature for two days and in the refrigerator for 5 days