Prepare the Gelatin: Pour 1/2 cup of water into the bowl of your stand mixer, fitted with the whisk attachment. Sprinkle the gelatin evenly over the water and let it sit for a few minutes to soften while you start the syrup.
Cook the Syrup: In a medium saucepan, combine the organic sugar, maple syrup, and blackberry purée. Cook this mixture over medium-high heat, stirring gently from time to time. Watch for the mixture to reach exactly 240°F (115°C) on a candy thermometer – this temperature is important for the perfect marshmallow texture.
Whip to Fluffy Perfection: Carefully pour the hot sugar syrup into the gelatin mixture in your stand mixer. Turn the mixer on high speed and beat for about 10 minutes. You'll see it transform into a wonderfully thick, fluffy, and pale cloud. Stir in the vanilla extract until it's just blended.
Set the Marshmallow: Lightly grease an 8x8-inch baking pan and line it with parchment paper, making sure to leave an overhang for easy lifting later. Gently scrape the fluffy marshmallow mixture into the prepared pan and smooth the top with a lightly oiled spatula. Let it cool completely at room temperature for at least 5 hours, or simply leave it overnight.
Dust and Cut: In a small bowl, whisk together the confectioners' sugar and potato starch. Once your marshmallow has completely set, sprinkle about two-thirds of this dusting mixture onto a clean work surface. Invert the marshmallow slab onto the dusted surface.
Final Touches: Using an oiled knife or pizza cutter, slice the marshmallow into uniform 1-inch cubes. Toss these cubes with the remaining dusting mixture to prevent them from sticking together. If you like, sprinkle a little of your favorite spice (like cinnamon) over them for an extra layer of flavor. Serve these delightful homemade marshmallows as a healthier treat, with hot cocoa, or on their own!