Heat the Cream and Water: In a saucepan, whisk together your cream and ¼ cup of water. Heat it until it just starts to bubble on the surface. Avoid a full boil.
1 cup heavy cream, ½ cup water
While the cream is heating, Prepare the Egg Mixture. In a mixing bowl, add three tablespoons of sugar and two large egg yolks. Whisk them together until combined.
2 large eggs, 3 Tablespoons granulated sugar
Temper the Eggs: Reduce the heat to low once you see bubbles in the cream mixture. Pour about a quarter of the hot cream into the egg yolk and sugar mixture, whisking immediately to temper the eggs. This prevents them from cooking too quickly.
Prepare Cornstarch Slurry: Mix ¼ cup of water with cornstarch in a separate bowl to create a slurry. Add the vanilla and this slurry to your custard mixture.
½ Tablespoon cornstarch, 1 Tablespoon vanilla paste, ½ cup water
Cook Until Thickened: Bring the mixture to a gentle boil, whisking constantly. The heat will activate the cornstarch, thickening the sauce. When the sauce coats the back of a spoon, turn the heat off.
Finish the Sauce: Once it starts to boil slightly, turn off the heat. Remove the pot from the stove and add a pinch of salt (about a quarter teaspoon). Whisk it in and let the sauce cool for a few minutes before transferring it to a container.
¼ teaspoon salt
Storage: Pour the sauce into an airtight container and store in the refrigerator until you are ready to use it.