In a bowl, combine chicken and buttermilk. Cover and refrigerate overnight. You can get away with just marinating for a couple hours, but overnight is best.
Combine flour, salt, pepper, chili, and paprika. Mix well.
When you're ready to get frying, pour some oil into a deep cast iron pan to a depth of about 2" or so, enough to barely float a piece of chicken, maybe with a little chicken island popping out the top.
Heat the oil to around 325f. You'll need a good thermometer with a clip so you can fix it to the side of the pan.
Take a thigh and dredge it in flour. Make sure you get a good coat.
Float your flour-coated chicken thigh into the hot oil. Give it 30 seconds or so and then carefully flip it over. Keep flipping your chicken every so often. Frying in shallow oil like this can mean that the region close to the pan can be a lot hotter than the surface. See that middle piece in the photo? I should have turned it sooner.
Before you move on to the next piece, keep an eye on the temperature. As you add a piece of chicken, the temp will drop, and if you let it get too low, your chicken will turn out kinda soggy and greasy. The cast iron helps moderate the heat a bit, but you need to balance the burner and the rate at which you add the chicken in order to keep the temp relatively steady. Aim for 325, but try your best to keep it between 300 and 350.
As each chicken thigh becomes golden crispy brown all around, transfer it to a plate that you've lined with paper towels to catch some of that excess grease.
To wrap things up, I like to fry a few sprigs of rosemary in the same hot oil for about 30 seconds, once I'm done frying my chicken. Fried rosemary serves as a nice garnish. The rosemary spines get crunchy and are quite tasty actually. It also creates a wonderful aroma.
Now, the toughest part of this can be the cleanup. When the oil has cooled, strain it into a sealable container. You can reuse the oil if you'll have a chance soon, but probably you'll need to get rid of it. Google for cooking oil disposal in your area. It should not be poured down the drain. You can throw it in the garbage, but it's better to get it recycled into biofuel.
