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, pressing on the solids with a spatula. Scrape off the back of the sieve to ensure you have 1/2 cup or 125 grams of puree. 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 fitted with whisk attachment. Beat on 8-10 min on high speed or 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. Place over medium heat, bring to a boil then reduce heat to a low boil and cook 5 minutes, whisking constantly until syrup has thickened and pours from a spoon without dribbling. Remove from heat.

With the mixer on the LOWEST speed, and as soon as agar syrup is off the stove, immediately pour it in a thin stream into whipped blackberry mixture. Avoid getting syrup on whisk and bowl. Once syrup is in, scrape down the bowl and beat another 2-3 min on high speed until stiff peaks form.

Transfer mixture to a piping bag fitted with a Wilton 1M large open star tip and pipe roses onto parchment-lined baking sheet, keeping the roses even in width/size.

Let homemade marshmallows stand in a draft free, low humidity, room temperature area (70˚F), uncovered for 6-12 hours, or overnight. Marshmallows will form a glossy film on top and will come off the parchment fairly easily. Once set, match 2 halves together then roll sandwiched marshmallows powdered sugar.