Instructions

for the pastry cream

Whisk egg yolks, 2 tablespoons sugar, and cornstarch in medium bowl until mixture is pale yellow and thick, about 1 minute; set aside. Heat milk, salt, and remaining 6 tablespoons sugar in medium saucepan over medium heat until simmering, stirring occasionally to dissolve sugar.
Gradually whisk half of milk mixture into yolk mixture to temper. Return milk-yolk mixture to saucepan. Return to simmer over medium heat and cook, whisking constantly, until mixture is thickened and 3 or 4 bubbles burst on surface, about 1 minute. Off heat, whisk in butter and vanilla. Transfer mixture to clean bowl, press parchment paper directly onto surface, and refrigerate until set, at least 3 hours.

for the genoise

Adjust oven rack to middle position and heat oven to 350 degrees. Grease bottom and sides of 18 by 13-inch rimmed baking sheet, line with parchment paper, grease paper, and dust with flour, tapping out excess.
Melt butter in small saucepan over low heat. Remove from heat and set aside. Sift flour and salt together onto large piece of parchment paper. Set aside.
Whisk eggs and sugar together in bowl of stand mixer until combined. Place bowl over pan of barely simmering water and heat egg mixture, whisking constantly, until warm to touch, about 110 degrees. Remove from heat and beat at medium-high until eggs are pale, cream-colored, voluminous, and form a thick ribbon of tiny billowy bubbles that falls from whisk and rests on top of batter for several seconds when whisk is held about 4 inches above egg mixture (this should take about 6 to 8 minutes). Beat in vanilla. Turn off mixer and transfer 1 cup of egg mixture to medium bowl and stir in reserved melted butter until combined; set aside. Grab two ends of parchment paper holding flour mixture and, with mixer at lowest speed, slowly sprinkle flour mixture into batter until just barely incorporated. Add melted butter mixture back to batter in stand mixer bowl and, with mixer at lowest speed, fold gently to incorporate, being careful not to deflate batter.
Holding bowl close to bottom of prepared pan, immediately pour batter into prepared pan, using offset spatula to push batter against sides and into corners of pan and smooth top. Bake until cake is deep golden brown, springs back lightly when pressed with a finger, and is beginning to pull away from sides of pan, about 25 minutes.
Immediately after baking, free cake edges from pan sides with knife and smooth sheet of parchment paper over cake top. Place second baking sheet on top of cake. Invert cake, then lift off baking pan. Let cake cool completely, then gently peel parchment paper off cake bottom, removing bottom crust. Reinvert cake, top side up, then peel off second sheet of parchment paper, removing top crust. Brush cake evenly with Grand Marnier; let stand about 10 minutes. Trim ¼ inch off each side of cake; discard trimmings. Using serrated knife, cut cake into 6 lengthwise and 8 crosswise strips to yield 48 approximately 2-inch squares. Set aside.

for the filling

Arrange 16 cake squares, fallen-domino style, around the bottom of  14- to 16-cup footed glass trifle dish (or equivalent glass bowl), placing 12 to 13 squares in ring against dish wall and remaining squares in center. Arrange about 1 cup each raspberries, strawberries, and blueberries, and a quarter of kiwi slices over cake squares. Spread a third of pastry cream, about 1 cup, over layer. Repeat layering cake, fruit, and pastry cream to make a total of 3 layers, reserving ¼ cup raspberries, ¼ cup strawberries, ¼ cup blueberries, and a quarter of kiwi slices for garnish. Cover bowl with plastic wrap and refrigerate for at least 6 hours but no more than 36 hours.

for the whipped cream

When ready to serve, beat cream and sugar in chilled bowl of stand mixer at medium speed to soft peaks; add vanilla. Continue to beat to barely stiff peaks. Using pastry bag, pipe whipped cream over top of trifle, or use spoon to spread whipped cream. Garnish with reserved fruit and serve immediately.