How to Make It
Prepare the Butterscotch Custard: Whisk together brown sugar, cornstarch, and salt in a large heavy saucepan. Whisk together half-and-half and egg yolks in a 4-cup glass measuring cup. Gradually whisk half-and-half mixture into brown sugar mixture. Cook over medium, whisking constantly, until mixture just begins to bubble, 8 to 10 minutes. Simmer, whisking constantly, 1 minute. Remove from heat; whisk in butter and vanilla until butter is melted. Spoon mixture into a bowl; place plastic wrap directly on top of mixture in bowl (to prevent a film from forming). Set aside to cool to room temperature, about 30 minutes. Chill at least 4 hours or up to 24 hours. (While Butterscotch Custard chills, prepare the Spice Cake and Vanilla Whipped Cream.)
Prepare the Spice Cake: Lightly coat 3 (8-inch) round cake pans with cooking spray, and dust with flour. Preheat oven to 350°F. Beat butter in bowl of a heavy-duty stand mixer on medium speed until creamy, about 2 minutes. Gradually add sugar, beating until light and fluffy, about 4 minutes. Add eggs, 1 at a time, beating until just blended after each addition. Stir together flour, baking soda, cinnamon, allspice, salt, and cloves in a bowl; add to butter mixture alternately with buttermilk, beginning and ending with flour mixture, beating on low speed until just blended after each addition. Add vanilla; beat on low speed until just blended. Pour batter evenly into prepared cake pans.
Bake cake layers in preheated oven until a wooden pick inserted in centers comes out clean, about 25 minutes. Cool cake layers in pans on wire racks 10 minutes. Remove from pans, and transfer to wire racks to cool completely, about 1 hour.
Prepare the Vanilla Whipped Cream: Beat cream and vanilla in bowl of a heavy-duty stand mixer fitted with whisk attachment on high speed until foamy, about 1 minute. With mixer running, gradually add powdered sugar; beat until stiff peaks form, 1 to 2 minutes. Cover and chill until ready to assemble trifle.
Assemble the trifle: Beat 1 cup heavy cream with an electric mixer on high speed until stiff peaks form, about 2 minutes. (Do not overbeat.) Gently fold into chilled custard. Horizontally slice 1/8 to 1/4 inch off the tops of each cooled cake layer using a serrated knife, creating an even, flat surface. Place 1 cake layer, cut side up, in an 18-cup trifle dish. (You will need to slightly push it into place to fit in bottom of dish.) Top cake layer with one-third of the custard mixture (about 1 1/3 cups); top custard with 1 1/3 cups whipped cream. Top whipped cream with 1 cake layer; repeat layering process with 1 1/3 cups custard and 1 1/3 cups whipped cream. Top with third cake layer and then with remaining custard. (Cover and refrigerate remaining Vanilla Whipped Cream until ready to serve.)
Cover assembled trifle; chill at least 4 hours or up to 24 hours. To serve, dollop remaining whipped cream over top. Sprinkle with ground cinnamon.