Steam the asparagus in a covered steamer over boiling water until crisp-tender (Or, put the asparagus in a microwave-proof dish and add about 1/4 cup water- Cover tightly and microwave on high for about 4 minutes- Drain well; set aside).
Melt 1 tablespoon of the butter in a small skillet over medium heat.
Add the corn and bell pepper; cook, stirring occasionally, until tender, about 5 minutes.
Add salt to taste; Set aside.
Melt the remaining 3 tablespoons butter in a Dutch oven over medium heat.
Add the mushrooms and leeks; cook, stirring occasionally, until tender but not brown, about 5 minutes.
Add the curry powder; stir for about 30 seconds.
Add the vegetable stock, milk, and asparagus.
Increase the heat to medium-high and stir occasionally as the liquid comes to a simmer, then reduce the heat to medium.
Stir together the cornstarch and cold water in a small bowl until smooth; stir into the soup.
Cook, stirring constantly, until the soup is smooth and slightly thickened, about 2 minutes.
Remove from the heat; Season to taste.
Top each serving with about 2 tablespoons of the corn-bell pepper mixture.