In a large bowl, whisk together yogurt, lemon, turmeric, 1 teaspoon garam masala, and cumin. Add chicken and toss until fully coated. Let it marinate for 15 minutes.
In a large pot or Dutch oven over medium-high heat, melt 2 tablespoons butter. Working in batches, add chicken and cook until golden all over, about 2 minutes per side. Transfer to a plate.
Reduce heat to medium and melt 2 tablespoons butter. Add the onion, jalapeños, and cinnamon stick. Cook until soft, about 5 minutes. Stir in the garlic, ginger, remaining teaspoon garam masala, and paprika and cook until fragrant, 1 minute more.
Add the tomatoes and bring mixture to a simmer. Return chicken to pot and toss to coat. Let simmer until the chicken is cooked through, about 8 minutes.
Stir in heavy cream and remaining 2 tablespoons butter.
Step 6 – Garnish with cilantro and serve warm over rice with naan.