Start by pouring cold water into a small bowl, then sprinkle the unflavored powdered gelatin in a single even layer over the water. Allow the mixture to sit undisturbed for 5-10 minutes, letting the gelatin absorb the water and swell.

In a medium saucepan, combine heavy cream, whole milk, strong brewed coffee, granulated sugar, and pure vanilla extract. Heat this mixture over medium heat, stirring gently, until it begins to simmer, which should take about 5-7 minutes.

Once the mixture is hot, remove the saucepan from the heat. Whisk in the bloomed gelatin until it's completely dissolved.

Carefully pour the smooth coffee mixture into 4-6 individual serving dishes or glasses. Allow the panna cotta to cool to room temperature, then cover with plastic wrap and chill in the refrigerator for at least 4 hours.

In a small saucepan, mix brewed coffee with granulated sugar over medium heat. Stir continuously until the sugar dissolves completely, about 3-5 minutes.

To serve, gently run a knife around the edges of the desserts if needed and invert onto plates or serve directly from the glasses. Drizzle the chilled coffee syrup over the panna cotta.