In the upper part of double boiler, off the heat, whisk together the egg yolks and sugar until frothy. Add the lemon juice, butter and water. Place over simmering water and stir constantly until it slightly thickens. Dissolve the cornstarch in the cold water. Stir into the lemon mixture. Cook until it thickens. Cover the surface with plastic wrap and refrigerate. To serve, spread the curd in the centre of the meringue. Garnish with seasonal berries.
