Preheat oven to 350 degrees F. Lightly butter or grease 8 mini (3.5-inch diameter) cast iron skillets. Arrange on a baking sheet.In a bowl, whisk together flour, baking soda, salt, espresso powder, and cinnamon until incorporated. Set aside.In a mixing bowl or the bowl of a stand mixer, beat butter and sugar until light and fluffy, 2 to 3 minutes. Mix in egg and vanilla, scraping down the side of the bowl as needed. Add dry ingredients and oats and mix until just incorporated. Stir in chocolate chunks.Spoon dough into 2-inch diameter balls, approximately 2 tablespoons or 1 3/4 ounces of dough per ball (I find a medium cookie scoop is the perfect portion for this). Drop balls into prepared skillets. Top with a few more chocolate chunks and a sprinkle of flake salt (if desired). Dough balls can also be frozen on a lined cookie sheet until frozen solid, then transferred to a zip top bag for later use.Bake for 14 to 16 minutes or until edges are golden and center is just set but not overcooked. If you like your cookies gooey, err on the side of underdone. If you like your cookies crispy, cook for a minute or two longer. Add one minute of cook time if cooking dough from frozen (do not thaw first).Let cool for 5 minutes before serving warm with homemade vanilla ice cream.