Preheat oven to 350ºF. Butter a 1 1/2 quart baking dish.
Combine the egg yolks and the sugar in a mixing bowl and beat until thick and creamy. Add the zest and lemon juice, melted butter, and milk. Beat well. Pour this mixture over the bread cubes and toss to combine.
Beat the egg whites until stiff peaks form. Gently fold the egg whites into the bread mixture. Pour into the buttered baking dish and bake at 350ºF for 30 minutes.
Serve warm with lightly sweetened whipped cream. This is also good served cold, but will deflate before serving. Serves 6.