Heat olive oil in a large skillet over medium heat. Add onion and carrots and sauté for 10 minutes or until vegetables begin to soften. Add curry paste, coconut milk, cinnamon, curry powder, and butter if you like, and let simmer until desired consistency. Season with salt and pepper. Add the lentils, and heat through, then remove from heat. Serve warm over rice, with a dollop of yogurt, if desired.