Tempering the egg-yolk mixture in this recipe with some of the hot liquid first keeps the eggs from cooking and curdling. Martha made this recipe in episode 507 of Martha Bakes.
In a medium saucepan, combine milk, 1/4 cup sugar, vanilla bean pod and seeds, and salt. Cook over medium heat until mixture comes to a simmer. Whisk in espresso powder.
In a medium bowl, whisk together egg yolks, cornstarch, and remaining 1/4 cup sugar. Whisking constantly, slowly pour about 1/2 cup hot-milk mixture into the egg-yolk mixture. Pour mixture back into saucepan, whisking constantly, and cook over medium heat, whisking, until it thickens and comes to a boil. Let boil for 2 minutes.
Remove from heat; add butter and stir until melted. Strain through a sieve into a bowl. Press a sheet of plastic wrap directly onto surface of pastry cream to prevent a skin from forming. Refrigerate until chilled, at least 2 hours or up to 2 days.
Just before using, whisk pastry cream until smooth. Whisk cream until soft peaks form; fold into pastry cream.