Instructions

for the pastry cream

Heat 3 cups milk in medium saucepan over medium heat until just simmering. Meanwhile, whisk sugar, cornstarch, and salt together in medium bowl. Whisk remaining ½ cup milk and egg yolks into sugar mixture until smooth. Remove milk from heat and, whisking constantly, slowly add 1 cup to sugar mixture to temper. Whisking constantly, return tempered sugar mixture to milk in saucepan.
Return saucepan to medium heat and cook, whisking constantly, until mixture is very thick and bubbles burst on surface, 4 to 7 minutes. Remove saucepan from heat; whisk in butter and vanilla until butter is melted and incorporated. Strain pastry cream through fine-mesh strainer set over medium bowl. Press lightly greased parchment paper directly on surface and refrigerate until set, at least 2 hours or up to 24 hours.

for the cake

Adjust oven rack to middle position and heat oven to 350 degrees. Lightly grease 18 by 13-inch rimmed baking sheet, line with parchment, and lightly grease parchment. Whisk flour, sugar, baking powder, and salt together in medium bowl. Whisk oil, water, egg, and vanilla into flour mixture until smooth batter forms.
Using stand mixer fitted with whisk, whip reserved egg whites and cream of tartar on medium-low speed until foamy, about 1 minute. Increase speed to medium-high and whip until soft peaks form, 2 to 3 minutes. Transfer one-third of whipped egg whites to batter; whisk gently until mixture is lightened. Using rubber spatula, gently fold remaining egg whites into batter.
Pour batter into prepared sheet; spread evenly. Bake until top is golden brown and cake springs back when pressed lightly in center, 13 to 16 minutes.
Transfer cake to wire rack; let cool for 5 minutes. Run knife around edge of sheet, then invert cake onto wire rack. Carefully remove parchment, then re-invert cake onto second wire rack. Let cool completely, at least 30 minutes.

for the fruit filling

Place 1½ cups strawberries, 1 cup blackberries, 1 cup raspberries, sugar, cornstarch, and salt in medium saucepan. Place remaining berries in large bowl; set aside. Using potato masher, thoroughly mash berries in saucepan. Cook over medium heat until sugar is dissolved and mixture is thick and bubbling, 4 to 7 minutes. Pour over berries in bowl and stir to combine. Set aside.

for the whipped cream

Using stand mixer fitted with whisk, whip cream, sugar, and 1 tablespoon sherry on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1 to 2 minutes.
Trim ¼ inch off each side of cake; discard trimmings. Using serrated knife, cut cake into 24 equal pieces (each piece about 2 1/2 inches square).
Briefly whisk pastry cream until smooth. Spoon ¾ cup pastry cream into trifle bowl; spread over bottom. Shingle 12 cake pieces, fallen domino–style, around bottom of trifle, placing 10 pieces against dish wall and 2 remaining pieces in center. Drizzle ¼ cup sherry evenly over cake. Spoon half of berry mixture evenly over cake, making sure to use half of liquid. Using back of spoon, spread half of remaining pastry cream over berries, then spread half of whipped cream over pastry cream (whipped cream layer will be thin). Repeat layering with remaining 12 cake pieces, sherry, berries, pastry cream, and whipped cream. Cover bowl with plastic wrap and refrigerate for at least 6 hours or up to 36 hours. Garnish top of trifle with reserved berries and serve.