Pat the thighs very dry. Season both sides using 1 tsp salt and ½ tsp pepper.
Add the thighs, skin side down, to a 10” cast iron pan. Place cold the pan on medium-high heat and cook undisturbed for 7-8 minutes, until the skin is deeply golden and crisp.
Flip and cook another 2 minutes. Remove the chicken to a plate.
If there is a great deal of excess fat, pour some of it off, leaving about 1 Tbsp in the pan.
Add the shallots and cook for 2–3 minutes until they're starting to soften. Add the garlic and cook another 30 seconds.
Add in the wine and simmer to reduce, about 2-3 minutes
Turn the heat down to low.
Stir in the Dijon, thyme, and maple syrup.
Slowly pour in the buttermilk, stirring gently to combine it with the Dijon and aromatics.
Add the remaining salt and pepper.
Nestle the thighs into the pan skin-side up, with the buttermilk coming about halfway up the chicken. Don't pour it over the skin.
Bring it just to a very gentle simmer.
Transfer the pan to a 325°F oven, uncovered, for about 35–45 minutes, until the chicken is very tender and is cooked through.
The buttermilk may separate a little. Don't panic! That's normal with heated buttermilk.
Remove and serve with crusty bread or mashed potatoes.