How to Make It
Prepare the custard: Whisk together sugar, cornstarch, salt, and eggs in a medium bowl until smooth, and set aside. Bring milk to a low boil in a medium-size heavy-bottomed saucepan over medium; remove from heat.
Temper eggs by slowly whisking one-third of the hot milk into sugar mixture until combined. Slowly add tempered egg mixture back to saucepan with hot milk, and whisk to combine. Cook over medium-low, whisking constantly, until mixture is thickened and just begins to bubble, 12 to 15 minutes.
Remove from heat; whisk in butter and vanilla. Transfer to a large bowl; place plastic wrap directly on warm custard to keep a film from forming. Refrigerate until chilled, at least 8 hours.
Whisk together chilled custard and buttermilk until smooth. Set aside.
Prepare the cake: Preheat oven to 325°F. Sift together flour, baking powder, baking soda, and salt; set aside.
Place white chocolate in a microwavable bowl, and microwave on 50% power until melted, 1 to 1 1/2 minutes, stirring every 30 seconds; set aside.
Beat together butter and 1 1/2 cups of the sugar in the bowl of a heavy-duty stand mixer on medium speed until light and fluffy, 4 to 5 minutes. Add melted white chocolate to butter mixture; beat on medium speed until incorporated.
With mixer on low speed, add egg yolks, 1 at a time, beating just until incorporated after each addition; add vanilla extract. Add flour mixture to egg yolk mixture alternately with buttermilk, beginning and ending with flour mixture.
Whisk egg whites in a separate bowl until frothy. Gradually add remaining 1/4 cup sugar, whisking until meringue is stiff but glossy. Gently fold meringue mixture into batter until incorporated. Transfer cake batter to a heavily greased and floured 13- x 9-inch cake pan. Bake in preheated oven until a wooden pick inserted in the middle comes out clean, 40 to 45 minutes. Cool cake completely on a wire rack before removing from pan, about 1 hour.
Prepare the trifle: Cut White Chocolate-Buttermilk Cake into 1 1/2-inch pieces, and arrange half of the cake pieces in bottom of an 18- to 20-cup serving dish. Spread half of the marmalade over cake pieces. Spoon half of the custard (about 3 cups) over marmalade. Layer sliced oranges in alternating colors tightly across top of custard. Repeat layers twice. Top with whipped cream; sprinkle with blood orange zest.