In a small sauce pan, cook the blueberries with the lemon juice and 1 teaspoon of the sugar until thickened, about 5 minutes.
Meanwhile, mash the strawberries in a small bowl with the remaining tablespoon of sugar.
Next, make the marshmallows: In a medium sauce pan, stir together the granulated sugar, corn syrup, salt, and 3 tablespoons of the water. Turn the heat to high and bring to a boil without stirring. Clip a candy thermometer to the edge of the pan, and boil until it reaches 238-degrees Farenheit.
Meanwhile, add the remaining 3 tablespoons of water to a large bowl and sprinkle the gelatin on top. Let sit for a few minutes without stirring.
hen the sugar syrup reaches 238, slowly begin to stream it into the gelatin mixture while constantly beating with a hand mixer (not a stand mixer). Do not splash the syrup on the edges of the bowl, or it will harden immediately. Go slow, and take your time.
Beat the mixture for a full 10 minutes.
Stir in the vanilla extract or vanilla bean seeds and mix until combined.
Spread the mixture into a 9 x 5" loaf pan that has been lined with parchment paper and greased very well with either cooking spray, butter or oil.
Next, swirl the blueberry and strawberry mixture into the marshmallows with a knife. Try to avoid adding the liquid from the strawberries and just aim for the berry pieces.
Let the mixture set uncovered for at least 3 hours.
When ready to cut, sprinkle the powdered sugar on a work surface. Dump the marshmallows out on the surface, and slice while dipping the knife in powdered sugar between cuts. Roll all of the edges of the marshmallows in the powdered sugar.
Store the marshmallows in an air-tight container at room temperature. They will keep for 2-3 days.
To make the s'mores, heat the marshmallow over an open flame until toasted to your liking. Sandwich between two graham crackers, using white chocolate as the 'glue.' Serve immediately.