Peel, core, and dice the apples. Add the apples, lemon juice, water, and sugar to a saucepan. Cook uncovered over medium-high heat for about 10 minutes, stirring occasionally to prevent sticking, until the apples are completely soft.
Pour the cooked apple mixture to a blender, blend it into a smooth purée.
Return it to the saucepan and cook uncovered until it becomes thick and concentrated. Measure out 180 g of apple purée.
Bloom 21g gelatin powder in 100 g cold water for about 10 minutes. Once fully bloomed, gently melt it over a hot water bath.
Meanwhile, combine the sugar, corn syrup, and water in a small saucepan. Cook over medium heat, stirring just until the sugar dissolves. Then stop stirring and continue cooking the syrup undisturbed until it reaches 116°C (240°F).
Place the concentrated apple purée and melted gelatin into the bowl of a stand mixer.
Turn the mixer to low speed. Slowly pour the hot syrup into the mixer. Increase to high speed and whip for 8–12 minutes until the mixture becomes thick, fluffy, glossy, and holds its shape.
If desired, add a few drops of green food coloring during the last minute of mixing.
Transfer the mixture into a parchment-lined pan and smooth the top. Let it set at room temperature for 6–8 hours, or preferably overnight.
Mix 60g powdered sugar and 60g cornstarch. Dust a cutting board generously with the powdered sugar and cornstarch mixture. Transfer the apple marshmallow to the cutting board, and cut it into cubes. Coat the apple marshmallows with the mixture of powdered sugar and cornstarch. Enjoy!