Preheat the oven to 400°F (200°C) and grease a baking sheet. In a large bowl, mix together the yogurt, garlic, ginger, turmeric, garam masala, chili powder and salt. Add the butter beans and stir to coat well, then transfer onto a baking sheet and bake for 15–17 minutes, until the beans start to look dry and a bit crisper.
Meanwhile, for the tikka curry, heat the oil in a large nonstick saucepan over medium heat. Fry the onion, garlic and ginger for about 2 minutes, until fragrant and the onions have softened. Add the bay leaf and spices and fry for another minute until aromatic. Next, add the tomato paste, chopped tomatoes and cilantro stems, then cover and bring to a boil. Lower the heat and let it simmer, covered, for 5 minutes.Add the beans when they’re ready, along with the yogurt, syrup and lemon juice. Bring back to a boil, then remove from the heat.