Prepare cherry puree. Place the berries in a small saucepan and add a cup of sugar. When the cherries boil, cook for about 5 minutes over medium heat until the berries soften. Remove the saucepan from the stove and puree the cherries using a blender or mortar. Add another 0.5 cups of sugar and begin to boil the puree over low heat for 3-5 minutes, remembering to stir. Then set the puree aside to cool.
Prepare the marshmallow mass. Place the agar covered with water on medium heat, bring to a boil and stir until completely dissolved. Then pour in the remaining sugar and cook the syrup for about five minutes, stirring. When it begins to stretch behind the spoon in a thin thread, remove it from the stove. Place the puree and egg white in a separate bowl, whisking the mixture constantly. Pour in the syrup in a thin stream, whisking continuously, until the mixture becomes light and thick.
Leave the marshmallow to harden. Quickly transfer the mixture into a pastry bag and pipe the marshmallow onto parchment. After that, leave it to harden.