Throw the rhubarb, white wine, sugar and lemon zest in a medium-sized saucepan. Stir until the rhubarb is fully encrusted with sugar. Place the pan over medium-high heat and bring the mixture to a boil. Reduce the heat to low and simmer uncovered for 15 minutes.

Once the 15 minutes has passed, fish out four pieces of rhubarb that are tender but not falling apart. Set the pieces aside and increased the heat to medium. Let the mixture bubble for 2-3 minutes, then take the saucepan off the heat.

Strain the mixture using a fine mesh strainer. Retain both the liquid and the solids. Let cool completely. When the rhubarb has cooled, puree the pulp using an immersion blender until very smooth.

Take 1/2 a cup of the rhubarb liquid and the milk and pour them in a small saucepan. Sprinkle 2 teaspoons of the gelatin over top and let the mixture stand for 10 minutes.

Once the 10 minutes has passed, add 1/3 of cup of the rhubarb pulp and the salt to the pan. Stir to combine. Place the saucepan over medium heat and heat the mixture to 135°F. Remove the saucepan from the heat and stir in the heavy cream and the vanilla. Transfer the panna cotta to a bowl.

Fill a larger bowl with ice and place the bowl with the panna cotta on top. Stir the mixture until it cools to 55°F. Transfer the panna cotta to a vessel with a spout and divide the mixture amongst four small ramekins. Place the ramekins in the fridge and chill for 2 hours.

When there is less than 15 minutes left in the chill time, take 2 tablespoons of the rhubarb liquid and sprinkle the rest of the gelatin over top. Let the mixture stand for 5 minutes.

Pour 1/2 cup of the rhubarb liquid in a saucepan and place it over medium-high heat. Bring it to a boil, then remove it from the heat. Stir in the gelatin mixture until completely dissolved. Set the mixture aside to cool slightly.

When the panna cotta has set, take the reserved rhubarb pieces and cut them in half lengthwise. Place the halves, cut-side-down, on top of the chilled panna cottas. Pour the warm rhubarb gelee mixture over top of the rhubarb pieces until they are halfway immersed. Return the ramekins to the fridge and chill for 1 hour more.

Take the panna cottas out of the fridge 10 minutes prior to serving to take the chill off. Serve with a cup of espresso.