Step 1

In a 6-quart stainless steel pot, melt butter over medium heat until foaming, then add carrots and onions. Cook, stirring occasionally, until butter starts to brown on the bottom, about 15 minutes. Add tomatoes, followed by chicken stock. Continue cooking, stirring occasionally, until quite thick, about 1 1/2 hours; adjust heat as needed to maintain a gentle simmer.

Step 2

Taste broth; if its flavor is too sharp or acidic, add 1/4 teaspoon baking soda and stir well (it will foam at first). Taste and repeat if needed. Purée soup with an immersion blender until smooth, or cool until no longer steaming before puréeing in a countertop blender. Stir in cream, then season with salt, pepper, and cayenne to taste. Rewarm to serve. Stir in basil just before serving. In a sealed nonreactive container, tomato soup will keep up to 1 week in the refrigerator or 6 months in the freezer.