All ingredients are suggestions and can be subbed out for whatever you've got that fits the role
Combine mustard, syrup, and vinegar to make a marinade, season with salt and pepper to taste
Mix chicken into marinade and set aside for an hour or more (or not, if you're going full "easy" mode)
Bake in a rimmed baking dish at 400F for 40-60 minutes, flipping half way through
Remove chicken from baking dish, pour juices into a pan, and simmer the juices to thicken into a sauce
As the sauce thickens (running a spoon/spatula through the sauce leaves a line for a moment), kill the heat, add the butter, and stir to melt and enrich the sauce; season with salt and pepper to taste
Pour the pan sauce over the chicken and serve
Notes:
This could probably be improved by sauteing aromatics in the pan (e.g. a shallot and some garlic) before adding the juices to reduce
Might be worth trying to not salt the marinade and instead salt the chicken, let it sit out for a bit, and pat dry before baking, to cut down on resulting juices
I did not trim the fat off of the thighs at all and it was fine