Spray a 9” pie plate with nonstick spray. Place bread cubes in the plate. Combine milk, eggs, sugars, vanilla, cinnamon and salt in a bowl. Whisk until well mixed. Add the blueberries to the mixture. Pour the mixture over the bread in the pie plate. Place a 9” dinner plate over the pie plate and place a heavy pot on top of that to keep it down. This will help to keep the bread down and enable it to soak up more of the liquid mixture. Allow the bread to soak for at least a half hour. Preheat the oven to 350 degrees F. Right before placing the bread pudding in the oven, spread the butter over the bread that is sticking up. Cook, uncovered for 45 minutes. Allow to cool for a while before serving. Serve warm as is, with a whiskey sauce, or with vanilla ice cream.