Preheat oven to 350°F (180°C).

1. Spoon the bread cubes into prepared casserole dish. Set aside.

2. In a saucepan, over low heat, heat milk, sugar, butter and salt just until butter has melted.

3. In a bowl, whisk eggs and vanilla. Whisk in the heated milk mixture until well blended. Spoon over bread cubes. Place in baking pan set on oven rack and pour in water to a depth of 1 inch (2.5 cm).

4. Bake, uncovered, in preheated oven for 40 to 50 minutes or until a knife inserted near the edge of the pan comes out clean and dry. Let cool on a wire rack.

5. Prepare the lemon sauce: In a medium saucepan, over medium heat, combine sugar, cornstarch and salt and mix to blend. Stir in water and zest and bring to a boil, cooking and stirring for 2 minutes. Remove from heat and add butter, lemon juice and food coloring (if using). Stir until well blended. Serve warm or cold over pudding.