1Line a 13x9x2 inch pan with parchment paper. Coat lightly with nonstick spray. Bring the water to a boil and add the juniper berries. Remove from heat and steep, covered, 15-30 minutes. The longer you steep, the stronger the juniper flavor.
2Strain and measure 120 grams (1/2 cup) of the juniper "tea" into a bowl and chill in the refrigerator or freezer until very cold. Measure another 120 grams (1/2 cup) of tea into a medium saucepan.
3Once the tea is chilled, pour it into the bowl of your mixer fitted with whisk attachment. Sprinkle gelatin evenly over the water, making sure all of it is wet. Let stand until gelatin softens and absorbs water, at least 15 minutes.
4Meanwhile, combine the sugar, corn syrup, salt, with the juniper tea in the saucepan. Stir over medium-low heat until sugar dissolves. Attach a candy thermometer to side of pan. Increase heat and bring syrup to boil. Boil, without stirring, until syrup reaches 235ºF, about 8 minutes.
5With mixer running at low speed, slowly pour hot syrup into gelatin mixture in a thin stream down the side of the bowl (avoid pouring syrup onto whisk, as it may splash).
6Once all the syrup is added, gradually increase speed to high and beat until mixture is white, fluffy, and very thick, about 15 minutes. Add in the vanilla extract if using and beat about 30 seconds longer. It will be voluminous & white, begin to pull away from the sides of the bowl, and will fall very slowly in a thick ribbon when the whisk is lifted when done.
7Scrape marshmallow mixture into prepared pan with a wet spatula (or coated with the nonstick spray). Work quickly as it gets harder to work with if it sits. And by harder I mean almost impossible. Smooth the top with wet spatula, and let stand uncovered at room temperature until firm, about 4 hours or overnight if covered loosely with parchment.
8Once the mallow set, sift juniper powder, salt, corn starch, and powdered sugar together in a bowl. Sift generous dusting of starch-sugar mixture onto work surface, forming rectangle slightly larger than 13×9 inches. Turn marshmallow slab out onto starch-sugar mixture; peel off parchment or foil. Sift more starch-sugar mixture over marshmallow slab.
9Coat large sharp knife (or cookie cutters) with water or nonstick spray. Cut marshmallows into squares or other shapes, keeping knife wet or coated. Toss each in remaining starch-sugar mixture to coat. Transfer marshmallows to rack, shaking off excess mixture. Store marshmallows in an airtight container. They will keep for about a week.