In a small mixing bowl, whisk together ½ c milk, cornstarch, vanilla and ⅛ teaspoon salt. Set aside.
Have the remaining milk ready in a measuring cup or small bowl.
In a medium saucepan, mix sugar and water and stir few times. Bring to boil over medium heat. Do not stir after boiling. If you need to, swirl it gently by moving the saucepan. Do not leave in unattended. Cook for about 5 to 6 minutes until it turns caramel color.
Remove from the heat. Add sea salt. Slowly add 1 and ½ cup of milk. Be careful as it will foam up. The caramel may turn dark but that's okay.
Place back on the stove. Cook to simmer until it thickens.
Gradually whisk in the cornstarch and milk mixture. Cook until it thickens.
Remove from heat and cool for few minutes in the saucepan. Transfer into a small mixing bowl and let cool to room temperature. Cover with saran wrap, making sure the wrap touches the surface of the pudding. Refrigerate until chilled (2 hours to overnight).
assembling the trifle:
When ready to serve, place cake pieces on the bottom of your serving dish and top with caramel pudding. Repeat as needed. Serve.
Notes
The amount of servings depends on how much cake you use for the trifle. I made 4 servings as seen on the picture.