Making the Filling

Pour the half-and-half into a 1½-quart heavy-bottomed saucepan, add the ½ cup sugar and stir to combine. In a 1-quart bowl, whisk the egg yolks and the remaining ½ cup sugar to combine. Add the flour and whisk to combine. Bring the half-and-half mixture just to a boil over medium heat. Remove and pour half of the hot liquid over the yolk mixture, whisking to combine. Pour the yolk mixture into saucepan and over medium heat, bring it to a boil again, stirring constantly. When it is thick and smooth, remove it from the heat, pour it into a 3-quart mixing bowl to cool, and cover the surface with plastic wrap to prevent a crust from forming on the cream. (This bowl allows room for folding in the whipped cream later.) Pierce the plastic with the tip of a knife to let steam escape so the custard cools faster. Refrigerate.

Assembling the Trifle

Split the cake evenly into three layers, each about 7/8 inch thick, using a 12-inch serrated knife in a sawing motion. (You may want to review the techniques for splitting a cake into layers.)

Lift the top two layers, one at a time, and set aside (this is easy because each layer is spongy and thick).

Stir the vanilla and rum into the cold custard. Whip the heavy cream in a 1½-quart mixing bowl to the Chantilly stage, or until it clings softly to the beaters. It should be thick enough to create swirls in the bowl but liquid enough to move if the bowl is tilted. Fold the cream into the custard mixture.

Place the bottom cake layer in the bowl and cover it with half the filling. Spread evenly with a rubber spatula. Center the middle layer on top, and cover it with the remaining filling. Center the last layer on top. Cover the dessert with plastic wrap and refrigerate.

Finishing the Trifle

Spoon 1 cup of Chocolate-Butter Glaze into a 1-quart bowl over a bowl of hot tap water (120 to 130 degrees). Stir until liquid and smooth.

Spread the glaze over the top of the dessert evenly with a metal or rubber spatula. Sprinkle Chocolate Flakes around the rim of the dessert.

Storing the Trifle

Refrigerate for up to 1 to 2 hours before serving.