Heat butter (or oil) over medium heat. Add leeks and cook for 5 minutes. Add garlic and cook for 3 more minutes.
Add all remaining ingredients to the pot except for the kale. Bring to a simmer. Cook for 10-15 minutes, until sweet potatoes are tender.
Turn off heat. Add 2 cups of the cooked soup to a blender or food proccesser. Blend until smooth. Add blended soup back to the main pot of soup. Alternatively you can use an immersion blender to partially blend the soup. If you have no access to a blender you can use a potato masher to mash the soup or just skip this step. This step serves to make the soup creamier.
Add in kale to soup and stir until wilted.
Divide out soup and serve.
Notes
This makes 3-4 servings of soup, depending on how hungry you are.If you use bone broth instead of vegetable broth this recipe will have 10 grams more protein per serving.*Mixing the miso paste with water allows it to dissolve better in the soup. I added mine to a small ramekin and mixed it with a fork until it was mostly combined with the water.