In a small bowl, sprinkle the unflavored powdered gelatin over the cold water. Let it sit for about 5-10 minutes until the gelatin softens.

In a medium saucepan, combine the heavy cream, whole milk, strong brewed coffee, granulated sugar, and pure vanilla extract. Heat over medium heat until warm.

Remove from heat and whisk in the bloomed gelatin until fully dissolved. Return to low heat if necessary.

Pour the mixture into serving dishes and refrigerate for at least 4 hours until set.

Prepare the syrup by simmering brewed coffee and sugar in a saucepan until sugar dissolves. Add Kahlua if desired and set aside to cool.

Drizzle coffee syrup over the set panna cotta just before serving. Optionally garnish with whipped cream or chocolate shavings.