Place butter in a 10 inch cast iron skillet, and put in oven for a few minutes to melt.
Once melted, remove pan and brush a bit of butter up sides of pan. Whisk in brown sugar and tahini. Let cool until just warm (a few minutes), then whisk in egg and vanilla.
Add flours, baking soda, baking powder, salt, cinnamon, and nutmeg. Stir together and then gently mash mixture with spatula. This may seem like its' not wet enough, but it will be - just keep combining. It will get to dough like texture. Then mix in chocolate chips.
Pat dough out to edges, and evenly across the skillet. Add a handful more chocolate chips to top of dough.
Bake for 14-15 minutes, until just browned and set. Remove from oven and set on wire rack to cool for 5-10 minutes so you don't burn your mouth. Then serve with vanilla ice cream.
Like all cookies, this one notices every minute it's in the oven. Keep an eye on it, as, you can go from an ultra-gooey center to something slightly cake-ier and more golden brown within just a few minutes.