Make the spice paste: place the cinnamon stick, green cardamom pods, cloves, peeled ginger, peeled garlic cloves, and jalapeño (seeds removed) in a spice/coffee grinder or blender and grind to a smooth paste. Set the paste aside.
Combine the marinade: in a medium bowl add the plain whole milk yogurt, the cubed chicken breasts, and 1 tsp salt. Add the prepared spice paste to the bowl and mix until the chicken is evenly coated. Cover and marinate at least 30 minutes in the refrigerator (longer if you have time).
Prepare the tomato-cashew puree: place the canned crushed tomatoes and the roasted cashews in a blender and puree until smooth. Set the puree aside.
Cook the chicken: heat a medium saucepan over medium heat and add the sunflower oil and all 4 tbsp butter. When the butter has melted and is hot, add the marinated chicken (including any remaining marinade) to the pan.
Sauté the chicken for about 10 minutes, uncovered, stirring every 2 minutes, until the chicken is mostly cooked through.
Add the tomato-cashew puree to the pan, stir to combine, lower the heat to low, cover, and cook for 10 minutes.
Stir in the heavy whipping cream (1/4 cup), the crushed dry fenugreek leaves (1 tbsp), and the sugar (1 tbsp). Mix until combined.
Cook covered on low for 5 minutes more, or until the sauce is hot, the chicken is fully cooked, and the butter begins to separate and float on top.
Remove from heat and serve.