In a small, heavy-bottomed pot, combine the sugar and the lemon juice with just enough water to stir and moisten to a sandy texture, being careful not to get any sugar on the sides of the pot. Cook over high heat, until caramelized to a dark brown color. Remove from the heat and, standing at arm’s length from the pot, add the 2 teaspoons of water. (Be careful not to burn yourself from the sputtering caramel.) Set aside for about 5 minutes, then pour the caramel into the bottom of 6 4-ounce styrofoam or unwaxed paper cups. Set aside the cups until caramel hardens.
To Make the Cream
Place gelatin in a small bowl with 2 teaspoons of cold water and allow to soften for 5 minutes.
In a medium saucepan, add cream and vanilla bean, sliced open. Gently stir the sugar into the mixture with a rubber spatula over low heat, and heat until hot to the touch. Remove from heat. Stir in the softened gelatin until it dissolves. Stir in the buttermilk and the lemon juice. Remove vanilla bean and strain through a fine mesh strainer. Scrape seeds out of vanilla pod and add to the cream. If using ground vanilla bean powder, add now.
Divide the mix equally among the caramelized molds. Refrigerate until the gelatin sets and the panna cotte are firm enough to unmold, at least 8 hours.
Invert the cup over a plate and puncture the bottom (now the top) with the tip of the paring knife. Invert the cup against the serving plate and it will slowly slide out.
To Make the Berry Sauce
Put sugar in a small pot with lemon juice and water. Cook over medium heat until sugar is dissolved, then bring to a boil. Allow to boil for about 5 minutes. Set aside and allow to cool to room temperature.
Put 1/2 cup raspberries in a blender with 1/4 cup of sugar syrup. Blend until smooth and strain through a fine strainer. Taste to adjust sweetening. Set aside until time to plate desserts.
Set out 6 chilled plates
Cut remaining blackberries and raspberries in half (if using) in a bowl with 1/2 teaspoon of syrup.
Invert the pannas over a plate one by one, piercing the bottom of the styrofoam containers and gently sliding them out onto the middle of each plate.
Drizzle a ring of raspberry sauce around each one and divide berries between the plates.