Make the paste: In a spice or coffee grinder, blend the cinnamon stick, cardamom pods, cloves, peeled ginger, garlic cloves, and jalapeno until a smooth paste forms; set aside.
Make the marinade: In a medium mixing bowl, combine the cubed chicken and the plain yogurt; stir to coat.
Add the prepared spice paste to the yogurt-marinated chicken and mix well; set aside to marinate briefly while you prepare the puree.
Make the puree: In a blender, puree the crushed canned tomatoes with the roasted cashews until smooth; set aside.
Heat the oil and butter in a medium saucepan over medium heat until the butter melts.
Add the marinated chicken (including any paste) to the pan and cook over medium heat for about 10 minutes, stirring every couple of minutes until the chicken is cooked through and no longer pink.
Stir in the tomato-cashew puree, lower the heat to a simmer, cover, and cook for 10 minutes to allow flavors to meld.
Stir in the heavy cream, crushed fenugreek leaves, salt, and sugar; mix to combine.
Cover and cook for another 5 minutes, or until the butter begins to float on top of the sauce and the sauce is slightly thickened.
Serve the butter chicken hot.
