Butter a glass 9 x13 pan. Spread half the bread cubes in bottom of pan. Evenly place cream cheese cubes over bread, then cover with the rest of bread. Whisk together eggs and milk, then pour over bread in casserole. Press bread down into the milk/egg mixture until all is moistened. Thinly slice butter and dot over top of casserole (alternatively, melt butter and drizzle over top). Sprinkle brown sugar over.
At this point you can bake it, or refrigerate up to 24 hours before baking for 40 minutes at 325*. When done, the whole casserole will be golden brown and will puff up somewhat like a souffle. If it jiggles in the center, it needs to bake a little longer. A toothpick inserted into the center should come out clean (unless you hit one of the cheese cubes). Let set until it firms up. Cut into squares and serve while still warm.
In a small pan, whisk together all sauce ingredients (before heating). Bring to a boil while stirring constantly to prevent lumps or scorching. Turn down heat and continue to bubble for a couple of minutes until clear and fragrant. Serve while hot.