Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9×13 inch baking dish with butter or non-stick spray.
Step 2: Place the bread cubes evenly into the prepared baking dish.
Step 3: In a large bowl, whisk together the eggs, granulated sugar, brown sugar, whole milk, heavy cream (for pudding), melted butter, vanilla extract (for pudding), cinnamon, and salt until well combined and the sugars are dissolved.
Step 4: Pour the custard mixture evenly over the bread cubes in the baking dish. Gently press down on the bread to ensure all cubes are submerged and soaking up the liquid. Let it sit for at least 15-20 minutes to allow the bread to fully absorb the custard.
Step 5: Bake for 45-55 minutes, or until the top is golden brown and the center is set. A knife inserted into the center should come out mostly clean. If the top browns too quickly, loosely tent with aluminum foil.
Step 6: Remove the bread pudding from the oven and let it cool slightly on a wire rack while you prepare the vanilla sauce.
Step 7: FOR THE VANILLA SAUCE: In a medium saucepan, whisk together the granulated sugar and cornstarch until no lumps remain.
Step 8: Gradually whisk in the heavy cream (for sauce) and milk until smooth. Place the saucepan over medium heat, stirring constantly.
Step 9: Bring the mixture to a gentle simmer, continuing to stir. The sauce will begin to thicken. Once it reaches your desired consistency (thick enough to coat the back of a spoon), remove it from the heat.
Step 10: Stir in the butter and pure vanilla extract (for sauce) until the butter is fully melted and incorporated. Taste and adjust sweetness if desired.
Step 11: Serve the warm Grandmas Old Fashioned Bread Pudding generously drizzled with the creamy vanilla sauce. Enjoy!