Step 1: In a medium saucepan, whisk together the sugar, cornstarch, and salt. Gradually whisk in the milk until smooth.
Step 2: Place the saucepan over medium heat and cook, stirring constantly, until the mixture thickens and begins to bubble.
Step 3: In a small bowl, lightly beat the egg yolks. Slowly whisk about 1 cup of the hot milk mixture into the yolks to temper them.
Step 4: Pour the egg mixture back into the saucepan and cook for another 2 minutes, whisking constantly. Remove from heat.
Step 5: Stir in the butter and vanilla. Transfer the custard to a bowl, cover with plastic wrap (pressing it directly onto the surface), and chill until cold.
Step 6: To make the ganache, heat the heavy cream in a small saucepan until it just begins to simmer. Pour it over the chopped chocolate and let sit for 5 minutes, then whisk until smooth.
Step 7: In a large glass trifle bowl, place half of the cake cubes. Top with half of the chilled custard.
Step 8: Repeat with the remaining cake and custard. Pour the chocolate ganache over the final layer of custard.
Step 9: Refrigerate the boston cream pie trifle for at least 4 hours before serving to allow the layers to set.