Preheat the oven to 350°F.
Make the sponge cake. In a small saucepan, warm the milk and 2 teaspoons of the butter together over medium-low heat.
With an electric mixer fitted with a wire whip, beat the eggs and 1 cup of the sugar on medium-high speed in a large mixing bowl until the mixture is pale yellow, thick, and tripled in volume, about 8 minutes. With the mixer on low speed, beat in the warm milk mixture.
Sift the flour, baking powder, and salt into a small mixing bowl. Add half the flour mixture to the egg mixture and blend thoroughly until smooth. Repeat with the other half. Add the vanilla and mix gently.
Grease a 17 X 12-inch baking pan or jelly-roll pan with the remaining 2 tablespoons butter. Sprinkle evenly with the remaining 2 tablespoons sugar. Pour the cake batter into the pan, spreading it evenly. Bake until the cake springs back when touched, about 15 minutes. Cool for about 2 minutes, then gently flip it out onto a large wire rack or a large sheet of parchment paper. Let cool completely.
Make the filling. In a medium-size mixing bowl, combine the sliced strawberries, Grand Marnier, and 1 cup of the sugar. Stir gently to mix. Cover and let stand for 1 hour.
Combine the mascarpone cheese with 1/2 cup of the sugar in a medium-size mixing bowl and mix well. Set aside.
Combine the egg yolks, the remaining 1/2 cup sugar, and the wine in a medium-size stainless steel mixing bowl and whisk until frothy. Set the bowl over a pot of simmering water and whisk until the mixture thickens, about 5 minutes. Add the melted butter in a steady stream, whisking constantly until it is well blended. Remove from the heat and continue whisking to cool slightly. Set aside and let cool for 10 minutes.
Whisk the mascarpone mixture, a little at a time, into the sabayon. Whisk in half the whipped cream.
Cut the cake vertically in half. Spread the strawberry mixture evenly over both halves.
To assemble, spread 1 cup of the sabayon evenly over the bottom of a deep glass dish (about 10 X 13 inches). Carefully place one piece of the cake on top of the sabayon and gently press down to remove any air pockets.
Spoon half of the remaining sabayon over the cake and spread evenly. Top with the remaining piece of cake and spread the remaining sabayon over the top. Cover and refrigerate for at least 12 hours.
When ready to serve, spread the top with the remaining whipped cream, then arrange the whole strawberries on the whipped cream. Spoon into dessert dishes.
Yield: 10 to 12 servings