1Preheat oven to 410 degrees F.
2Combine the cold water in a saucepan with the salt, sugar, and butter. Bring to a boil over low heat. When the mixture is boiling, remove from the heat. Pour all the flour in at once. Stir vigorously with a wooden spoon until the flour is completely absorbed. Put the pan back over low heat for 1-2 minutes while continuing to stir with a wooden spoon in order to dry out the dough. As soon as the dough sticks to the bottom of the pan, put it in a bowl. Let it cool for a few moments then add the eggs one at a time, completely incorporating the eggs after each addition.
3Put the dough into a size 8 piping bag and make small heaps of dough on a baking sheet covered in parchment paper. Make sure to space your pastry widely. Sprinkle with powdered sugar. Bake for 10 minutes, then turn the temperature down to 350 F for 15 minutes. After baking, turn off the oven and open the door slightly. Leave the puffs in the turned-off oven for 10 minutes.
4Mix the sour cherry puree with the agar in a saucepan. Boil for 1-2 minutes.
5Mix the cornstarch and sugar in a bowl and dilute it with a small amount of the cold milk.
6Bring the remaining milk to a boil. Pour the hot milk into the bowl with the cornstarch mixture. Mix and then pour the mixture back into the pan and return it to the heat. Add the sour cherry puree and let it thicken for a few moments. Remove from the heat and cover the surface with plastic wrap to avoid a film from forming.
7In another bowl, whisk the egg whites and gradually add the confectioner's sugar until it achieves the desired texture. Then add drops of red food coloring.
8Fill the puffs with the cherry cream by punching a hole in the bottom and using a piping bag to add the filling.
9Dip the the top of the puff in the icing while rotating it. Decorate with sprinkles.