Bloom the gelatin: Sprinkle gelatin over the cold water in a small bowl.
Let sit 5–10 minutes until spongy.
Heat the base: In a saucepan, combine cream, milk, sugar, and a pinch of salt. Warm over medium heat until steaming and sugar dissolves. Do not boil.
Add chocolate: Remove from heat.
Stir in chopped chocolate until completely smooth. Add vanilla.
Incorporate gelatin: Stir the bloomed gelatin into the warm mixture until fully dissolved. If needed, return to very low heat briefly to help it melt.
Do not simmer.
Strain and portion: For ultra-smooth texture, pour through a fine strainer into a pitcher. Divide among 6 small ramekins or glasses.
Chill: Let cool 15 minutes at room temperature. Cover and refrigerate 4 hours, or until set with a gentle wobble.
Make the espresso caramel: In a light-colored saucepan, combine sugar and 2 tablespoons water.
Cook over medium heat, swirling (no stirring) until amber, 5–8 minutes. Carefully add espresso; it will bubble. Whisk in butter, then cream, and a pinch of flaky salt.
Simmer 30 seconds, then cool to room temp.
Top and garnish: Spoon a thin layer of espresso caramel over each panna cotta. Finish with a few flakes of sea salt and optional berries or shaved chocolate.