To make strawberry puree, in a medium saucepan, cook fresh or frozen strawberries with 1 tablespoon of sugar over medium high heat for 10-15 minutes. (15 minutes if you’re using frozen berries.) Mash the fruits as they cook.
Run the mixture through a sieve. (Note: You should have about 250g of puree.) Cool completely and then chill in the fridge. It’s best if you make it a day in advance.
Prepare the tools before you begin: set a piping bag with decorative tip in a tall glass, so it's easy to fill with batter. Line 2 baking sheets with parchment paper or silicone mat.
To make sugar syrup, in a medium saucepan, combine 200g of sugar, agar and water. Stir everything well and cook over medium heat until 240°F, about 10 minutes. (TIP: Don’t stir it after the initial stir.)
When the sugar syrup reaches 200°F, start making the meringue. (Next step.)
To make the meringue, in a mixing bowl with a whisk attachment, combine strawberry puree with egg white and salt. Beat the mixture on low speed until nice and foamy.
Once it’s foamy and pale, increase the speed to medium and start adding the remaining 100g of sugar one tablespoon at a time. Whip the meringue until soft peaks form.
When the syrup reaches 240°F, remove from heat and let the bubbles subside, no more than half a minute.
To make marshmallows, slowly pour the sugar syrup into the meringue, while mixer is running on medium high speed. (NOTE: Don’t pour the hot syrup into the whisk, aim it toward the side of the bowl, so it doesn’t splatter.)
Don’t scrape the hot syrup from the saucepan.
Once all the syrup is poured in, increase the speed to medium high and whip until hard peaks form, 3-5 minutes.
Immediately transfer the mixture into the prepared piping bag with decorative tip. (I love using Wilton 1M for this.)
Pipe 2-inch round rosettes (about 1 inch tall) on the prepared baking sheets. Let them set for at least 8-12 hours. I usually make this at night, so they set overnight.
Then dust the marshmallows with plenty of powdered sugar and enjoy!