Cut 2-inch holes out of the bottom of a large tin foil roasting pan in neat rows, leaving 1 to 2 inches in between the holes. Invert the pan and place paper cooler cups in the holes to hold them upright during freezing.
In a large heavy-bottomed saucepan combine the lemon juice, sugar, yolks, egg, and butter. Over medium heat, whisk the mixture constantly until thickened. Strain it into a bowl set over ice and chill it, stirring to thicken slightly. Fold in the whipped cream and pour or pipe into paper cones set in the prepared tin foil roasting pan. Freeze overnight. Peel off the paper and serve the cone flat side down and serve with berries.
In a small saucepan bring water, sugar and vanilla bean to a boil. Add blackberries and turn off the heat. Fish the berries out of the hot liquid with a slotted spoon, and transfer them to a plate in a single layer and allow to cool.
Yield: 1 cup