1. In a pan, combine milk and salt. Warm over medium heat until milk is just below a simmer. In a bowl, whisk together sugar, cornstarch and eggs until smooth.
2. Whisking constantly, slowly pour 1/3 of milk into sugar mixture. Whisk mixture back into pan with remaining milk; return pan to medium heat. Cook, whisking constantly, until mixture begins to thicken, about 5 minutes. Pour through a fine-mesh sieve into a clean bowl; whisk in butter and vanilla. Cover with plastic, pressing it directly on surface of custard to prevent a skin from forming. Refrigerate until completely cool, about 1 hour.
3. When custard is cool, arrange cake slices on a work surface. Spread one side thinly with jam. Line bottom of a glass trifle dish with 1/3 of pound cake slices, jam side up. Top with 1/3 of strawberries, and drizzle with 1/3 of triple sec. Spread with 1/3 of custard. Repeat layering until you have used all cake, berries, custard and triple sec, ending with custard. Refrigerate until ready to serve.