Prepare Ultimate Vanilla Custard: Whisk together egg yolks, sugar, cornstarch, and salt in a medium-size heatproof bowl until smooth. Set aside.

Halve vanilla bean pod lengthwise. Using the back of a paring knife, scrape seeds from pod halves into a medium saucepan; add scraped pod halves and milk. Bring to a simmer over medium, stirring occasionally. Remove from heat.

Gradually ladle half of hot milk mixture into yolk mixture, whisking constantly. Return egg yolk mixture to remaining milk mixture in saucepan. Cook over medium, whisking constantly, until mixture boils and thickens, 8 to 10 minutes. Remove from heat; stir in butter and vanilla extract until combined. Pour through a fine mesh strainer into a large heatproof bowl. Remove and discard vanilla pod halves. Cover custard with plastic wrap pressed directly onto surface. Chill until cold, about 2 hours. Store, with plastic wrap still pressed against surface, in refrigerator up to 3 days.

While Custard chills, prepare Vanilla Chiffon Cake: Preheat oven to 350°F. Coat a 13- x 9-inch metal baking pan with cooking spray; line with parchment paper, leaving a 2-inch overhang on long edges, and recoat with cooking spray. Set aside.

Beat egg whites and cream of tartar with a stand mixer fitted with a whisk attachment on medium speed until foamy, 1 to 2 minutes. Increase mixer speed to high; gradually add ½ cup of the sugar, beating until stiff, glossy peaks form, 2 to 3 minutes total. Transfer mixture to a medium bowl; set aside. (Do not wipe mixer bowl or whisk attachment clean.)

Add egg yolks, vegetable oil, vanilla bean paste, vanilla extract, and remaining ½ cup sugar to mixer bowl; beat with whisk attachment on medium-high speed until pale yellow, about 3 minutes. Sift together flour, baking powder, and salt in a medium bowl. Reduce mixer speed to low; gradually add flour mixture, beating just until combined, about 1 minute. Gradually and gently fold in egg white mixture just until combined.

Pour batter into prepared pan; gently and evenly spread batter to edges. Bake until center of cake springs back when pressed, 15 to 17 minutes. Remove from oven. Let cool completely on a wire rack, about 1 hour. Use immediately; store, covered, at room temperature up to 1 day; or freeze up to 1 month.

Cut cake into 1 ½-inch cubes (to yield 10 cups); set aside.

Prepare Vanilla Whipped Cream and assemble Trifle: Beat cream, vanilla bean paste, and granulated sugar in a large bowl with an electric mixer on medium-high speed until medium peaks form, 3 to 4 minutes. Set aside.

Spoon one-third of the custard (about 1 ⅓ cups) into bottom of a 14-cup trifle dish or bowl. Top with one-third of the cake cubes (about 3 ⅓ cups). Reserve 3 cups of the whipped cream for topping the trifle; top the cake-cube layer with half of the remaining whipped cream (about 1 ⅓ cups). Repeat layers twice, omitting final whipped cream layer. Cover and chill at least 2 hours or up to 12 hours.

Just before serving, uncover and top with reserved whipped cream and crushed meringue mushrooms, if using.