In a medium saucepan, bring blackberries, 1/2 cup sugar and 1 Tbsp lemon juice to a simmer. Mash berries and simmer uncovered 10 min then strain through a fine sieve. Chill the puree over an ice bath until cooled.

Once puree is fully chilled and slightly thickened, combine it with 1 egg white in the bowl of a stand mixer. Beat on high speed for 8-10 min until mixture is thick and stiff peaks form.

In a medium saucepan, stir together 1/3 cup water, 2 tsp agar agar and 1 cup sugar. Bring to a boil, reduce to a low boil, and cook for 5 minutes while whisking constantly.

With the mixer on the lowest speed, pour the agar syrup in a thin stream into the whipped blackberry mixture. Scrape down the bowl and beat for another 2-3 min on high speed until stiff peaks form.

Transfer mixture to a piping bag with a large star tip and pipe roses onto a parchment-lined baking sheet. Let stand uncovered for 6-12 hours until marshmallows form a glossy film.

Once set, match 2 halves together and roll the sandwiched marshmallows in powdered sugar.