In a small nonreactive* heavy saucepan, combine egg yolks and sugar; let stand for 5 minutes. Add the lemon juice; mix well. Cook and stir constantly over medium heat 7 to 8 minutes or until thickened and bubbly and an instant-read thermometer registers 180°F.
Once the curd is very thick, remove from heat and strain through a fine mesh strainer into a small bowl. Let mixture cool about 20 minutes or until an instant-read thermometer registers 140°F. Stir in the butter and, if using, the vanilla paste or vanilla. If using vanilla bean, using the tip of a paring knife, slit vanilla bean lengthwise down the center. Using the knife, scrape the seeds from the vanilla bean into the lemon curd.
Cover surface of the curd with plastic wrap. Chill for at least 1 hour. (Store, covered, in the refrigerator up to 1 week.)
In a chilled metal mixing bowl, beat whipping cream with an electric mixer on medium speed, beating just until soft peaks form. Stir a small amount of the whipped cream into lemon curd to lighten. Gently fold in the remaining whipped cream until all is the same color.
To serve, using a small sharp knife, gently loosen edge of Easy Tart Shell from sides of pan; remove sides from tart pan. Place tart shell on a serving plate. Spoon the lemon curd mixture into tart shell. Top with berries. Cut tart into wedges.
Because acidic foods react to aluminum pans, use a stainless steel, glass, or enamel pan to prepare curd.
Let stand at room temperature 15 minutes. Unroll piecrust. Ease pastry into an ungreased 11-inch tart pan with a removable bottom. Press pastry into pan; trim edges. Prick bottom and sides of pastry. Line with a double thickness of foil. Bake in a 450°F oven for 8 minutes. Remove foil. Bake for 5 to 6 minutes more or until lightly browned. Cool completely in pan on a wire rack.