Add butter to a large stockpot. Turn the heat on medium. Once the butter melts, add the onion, carrot and 1/2 teaspoon salt. Sweat the veggies until slightly softened, about 4-5 minutes. 

Add tomatoes, chicken broth and remaining salt. Bring to a boil and reduce to a simmer. Simmer for 25 minutes or until the carrots are super soft and the tomatoes have broken down. 

Carefully transfer to a blender to puree. Puree until smooth. Transfer back to the pot and season to taste with salt and pepper.