Slice the onion into ½ inch slices, separate the rings. Place in a large bowl, cover with buttermilk. Refrigerate for 1 hour and up to overnight.
In a large bowl add 1 cup flour (reserve the other cup), cornstarch, baking soda, chili powder, brown sugar, stir to combine. Stir in the beer to make a smooth batter.
Add the remaining flour to a small bowl. Stir together the salt and panko in a separate bowl.
Heat three inches of oil in a pot over medium high heat. Use a deep fry thermometer to maintain 350 degrees.
One at a time remove the onion slices from the milk, dredge in flour until well coated, dip in the batter, then add to the panko to gently coat (if panko bowl becomes too saturated with the dip, toss it and fill the bowl with fresh panko). Fry on both sides until golden brown, about 1 minute per side.
Remove from oil, place on a wire rack to cool.