Melt 2 tablespoons butter in medium sized stock pot, and saute onion and celery until tender.
Add broccoli and broth, cover and simmer for 10 minutes. Using a potato masher, mash the soup until it is the consistency you would like (you may blend it, but then you don't end up with any chunks of broccoli, and I like to have some chunks).
In a small saucepan, over medium-high heat, melt 3 tablespoons butter, stir in cornstarch and add milk. Stir until thick and bubbly, and add to soup. Season with pepper and serve.