In a small Dutch oven or heavy soup pot, add the mashed potatoes, stock, salt and pepper (and optional cayenne). Cook over low heat until the potatoes and stock have come together
To make soup super-smooth, remove the pot from heat, and use an immersion blender at this point to purée the soup.
Add the yogurt and 1-1/2 cups grated cheese. Gently stir until the cheese melts, and the soup turns a pale orange. Taste, and add more salt and pepper if needed.
Ladle into individual serving bowls, and top with a sprinkling of black pepper and a bit more cheese. Serve hot.