Pour the cold water into a large bowl and sprinkle the gelatin on top. Let sit for 5 minutes.
Pour the cream and sugar into a medium-sized pot and set to medium-high heat. Stir to dissolve the sugar and heat until the mixture starts to bubble. Remove from the heat and add to the gelatin. Whisk to combine.
Add the milk and enough red food colouring to reach your desired shade of red. Whisk to combine. Pour the mixture through a sieve.
Set the heart-shaped molds into bowls to keep them level, then fill the molds with the panna cotta. Place in the fridge and chill overnight, until the panna cotta has set.
To make the syrup, dye the water a deep red with some food colouring. Pour the water, sugar and vanilla extract into a small pot, set to medium-high heat.
Bring the mixture to a boil, stirring to dissolve the sugar. Reduce the heat to medium-low and simmer for 25-30 minutes, until the syrup reduces to about ½ cup. Remove from the heat and cool for 5 minutes, then transfer to a jug to serve.
Turn the hearts out onto serving plates. If the panna cotta is sticking to the molds, wrap a warm, damp dishcloth around the molds to slightly warm the mold.
Pour the bloody syrup on top and enjoy!