In a small bowl, sprinkle the powdered gelatin over the cold water. Let it sit for 5–10 minutes until it has absorbed the water.
In a medium saucepan, combine the heavy cream, whole milk, sweetened condensed milk, kosher salt, and vanilla extract. Place the saucepan over medium heat, stirring occasionally, until the mixture is hot and smooth and the condensed milk has fully melted. Whisk in the espresso powder until dissolved. Do not let it boil.
Add the bloomed gelatin to the mixture. Stir gently until the gelatin is completely dissolved.
Evenly divide the mixture among 5 serving dishes or ramekins. Allow them to cool slightly at room temperature.
Place the dishes in the refrigerator, uncovered, and let them chill overnight (at least 8 hours) until fully set.
Once set, serve the panna cotta directly from the dishes. Top with salted caramel sauce and whipped cream if desired. Caramel sauce recipe linked in notes below.
Storage instructions in notes below.