Combine all marinade ingredients in a blender. Coat the chicken in the marinade and marinate overnight or for at least 4 hours.
Preheat the oven to 350°F.
Place the potatoes in a large saucepan and cover with cold water by two inches. Add a teaspoon of salt and bring to a simmer. Cook until fork-tender, then drain and let the potatoes dry for 10 minutes.
Combine the butter, cream, brown sugar and maple syrup in a small saucepan over medium heat. Whisk constantly and heat through, being careful not to let the bottom scorch.
Return the potatoes to the pot and mash with a potato masher. Fold in the cream mixture, along with the sour cream, pecans, vanilla and lemon juice. Season with salt and pepper.
Remove the chicken from the marinade, discard marinade and season chicken with salt and pepper. Roast on a baking sheet until cooked through, about 22-25 minutes.
For the chipotle sauce, melt the butter in a medium saucepan. Add the garlic and cook over low heat until softened, about 5 minutes. Add the flour, coriander, cumin and oregano and whisk to make a thick roux. Cook over medium heat until the flour turns light golden, stirring constantly, about 10 minutes.
Add the cream and bring to a boil. Reduce to as simmer and cook for 20 minutes. Fold in the chipotle purée and season with salt and pepper.
Toss the broccolini with the oil, garlic, salt and pepper on a baking sheet. Roast for 10-12 minutes, until crisp-tender.
To serve, divide the mashed sweet potatoes between plates. Top with chicken and broccolini and drizzle with sauce.