Beat egg whites, cream of tartar and salt in a large bowl, with an electric mixer, until soft peaks form. Beat in sugar, 1 tablespoon at a time, until stiff, glossy peaks form. Beat in cornstarch, vinegar and vanilla just until blended.
On a baking sheet lined with parchment paper or foil, spread meringue into a rectangle, with a raised edge, and a slight indentation in the center.
Bake in a preheated 250 degrees F (120 degrees C) oven until firm to the touch, about 1 hour 15 minutes.
Remove from oven and allow to cool.
When ready to serve, place meringue on a serving plate. Cut strawberries into halves or slices, if large.
Whip cream until stiff peaks form, adding powdered sugar (orange liqueur optional).
Spread whipped cream over meringue, leaving edge of meringue visible. Top with strawberries. Cut into squares to serve.