Slit the vanilla pod open lengthwise, and scrape out the seeds with the flat edge of a knife into your saucepan. Add both the seeds and the pod, as well as the milk, into your pan. Bring to a boil over med heat, then remove from heat and set aside.
Beat the sugar, eggs yolks, and cornstarch together in a large bowl.
Gradually pour in a bit of hot milk at a time, and continue whisking, until all the milk is incorporated. Return the mixture to the saucepan, and cook over a low to medium heat, stirring constantly, until it thickens. *The mixture may go lumpy partway through, but continue stirring and it will smooth out again.
Once thickened, lower the heat, and continue cooking for 2 minutes.
Remove from the heat, and mix in the butter, followed by the rosewater.
Transfer to a heatproof bowl, and press a sheet of cling film (saran wrap) to the surface of the cream, and leave to cool. *It's important that the sheet is directly on the surface of the cream, and not just on the bowl - otherwise, your cream will form a skin.
Whip your heavy cream (aka double cream) to soft peaks. Once your pastry cream is cool, stir it vigorously, and then fold in your whipped cream gently. Taste and adjust the rose flavor if necessary.
Transfer the mixture to a piping bag fitted with a 5mm nozzle (for filling your eclairs). Make a small hole in the tip of your eclair, insert the nozzle, and fill the eclairs generously. *See video for reference on how to do this.