Prepare the Custard: Whisk together sugar, flour, and salt in a saucepan. Whisk in milk and egg yolks. Cook over medium-high, whisking constantly, until mixture begins to bubble, about 10 minutes. Continue to cook until thickened, about 4 more minutes. Remove from heat; whisk in butter, orange juice, vanilla, and bitters until smooth. Transfer to a heatproof glass bowl. Let stand at room temperature until slightly cool, 30 minutes, stirring occasionally. Place plastic wrap directly on surface of custard; chill until cold, at least 4 hours. 

Prepare the Cherries: Stir together cherries, preserves, and whiskey in a bowl. Set aside at room temperature.

Prepare the Trifle: Combine whiskey and sugar in a small saucepan. Bring to a boil over medium-high, whisking occasionally, until sugar dissolves. Remove from heat; cool to room temperature, 20 minutes. Place cake slices in a single layer on a work surface. Brush with cooled whiskey syrup; let stand 5 minutes. Cut cake into cubes.

Layer one-third of the cake cubes, one-third of the Custard, and one-third of the Cherries in a 4-quart trifle dish. Repeat layers twice. Top with whipped cream and orange peel twists. Serve immediately, or chill up to 12 hours.