For the watermelon juice: use pre-made watermelon juice like WTRMLN water, or puree slices of watermelon in a blender. Pour the juice through a mesh sieve before using. Discard pulp and seeds. Refrigerate any leftover watermelon juice for a refreshing treat later.
To make the pastry cream, heat the milk in a medium sized saucepan over medium to medium high heat. Once the milk begins to steam, remove it from the heat. Do not let the milk boil.
Once removed from heat, stir in the ½ cup of watermelon juice and let mixture stand for 10 minutes.
Meanwhile, whisk the eggs and sugar together in a bowl for 60 seconds.
Combine the salt and cornstarch, then add to the sugar-egg mixture. Whisk for 5 minutes.
Temper the egg mixture by spooning in the hot watermelon milk mixture, one tablespoon at a time, while constantly whisking the egg mixture. Do this SLOWLY until all of the milk is combined into the eggs. Pour the whole mixture back into the saucepan.
Over medium heat, keep whisking for 5 minutes. Do not let it boil. Once the mixture has thickened a bit, remove from the heat and add in the butter that has been cut into smaller pieces.
Whisk until the butter melts and let mixture stand for a few minutes. Pour into a glass container and tightly close the lid. Refrigerate for 20 minutes at least.
When ready, pour ½ cup of the pastry cream into a piping bag fitting with a medium sized tip (I used Wilton ##). Poke the piping tip into the bottom of the croissant and squeeze to fill with cream. You can poke 2 holes in each side of the croissant bottom if desired. Set each croissant aside on a plate. OR, cut each croissant in half lengthwise with a bread knife. Spread pastry cream over the middle of the croissant, and sandwich the glaze between the two halves.
Mix together the glaze ingredients in a small bowl. Use a spoon to drizzle the glaze over the filled croissants.