Place a well seasoned cast iron skillet on stovetop over medium heat, add cold butter
Let butter melt, coating entire skillet, tilting to coat sides
Turn off heat, add sugars and vanilla, blend with a spoon until well combined
Allow mixture to cool for a few minutes, add eggs, mix until well incorporated
Stir in flour, chia seeds, soda and salt, scraping the sides and bottom of skillet to leave no dry flour
Stir in chocolate chips (reserve ¼ cup chocolate chips to sprinkle over brownie top)
Bake in oven on middle rack for 15-20 minutes
Notes
Over stirring the warm mixture will melt the chips and produce a brownie, if you prefer a chocolate chip cookie, gently stir until chips are just incorporated. Serve with vanilla ice cream or a big glass of milk