Preheat oven to 350 degrees F.
Make shell: Blend ground shortbread, sugar, and butter in a food processor until mixture begins to clump together. Press onto bottom and up side of tart pan. Bake in middle of oven 15 minutes, then cool in pan on a rack. Make ginger cream: Sprinkle gelatin over milk in a small bowl and let stand 1 minute to soften. Transfer to a saucepan and cook with ginger, sugar, lemon juice, salt, and 1 cup heavy cream over moderate heat, stirring, until sugar and gelatin are dissolved. Cool to room temperature, about 1 hour. Whisk sour cream in a large bowl until smooth. Beat remaining cup heavy cream in another bowl with an electric mixer until it just holds soft peaks and fold into sour cream. Gently fold ginger milk into whipped cream until combined well. Pour into crust and chill until set, at least 8 hours. Dust tart with confectioners' sugar and serve with berries.
Recipe courtesy of Gourmet Magazine