In a medium saucepan bring milk, vanilla bean and scraped seeds just to a boil, stirring to prevent film from forming.
In a separate large bowl, whisk together sugar, cornstarch and a pinch of salt. Add 4 egg yolks and whisk until smooth, creamy and lightened in color - it takes a couple of minutes but it will get there!
Gradually (so the eggs don’t curdle), while whisking constantly, add hot milk in a steady stream until all of it is incorporated. Pour mixture back into the saucepan and bring to a boil while whisking constantly then whisk for another 30 seconds until mixture is thick and pudding-like in consistency.
Transfer pastry cream to a medium bowl (whisk in vanilla extract, if you’d like). Cut butter into pieces and quickly whisk into the custard until fully incorporated.
Cover with plastic directly over the surface of the cream, let it cool slightly, then refrigerate 30 minutes or until cool.
With a small pastry tip, poke 2-3 holes through the bottom of each cooled pastry. Pipe cream inside, scraping off excess.
Refrigerate eclairs while making chocolate glaze.