For the marshmallows: Pour 1/2 cup cold water into the bowl of a stand mixer and sprinkle the gelatin over the top. Soak for 10 minutes. Line a 9-by-9-inch baking pan with plastic wrap and oil it.
In a saucepan, combine the sugar, corn syrup and 1/4 cup water and bring to a boil. Boil for 1 minute, then pour into the gelatin and mix on high speed. Add the salt and beat for an additional 12 minutes, then add the vanilla.
With a lightly-oiled spatula, scrape the marshmallow mixture into the prepared pan and spread evenly. Oil another piece of plastic wrap and use it to press the mixture into the pan. Let sit at room temperature for 2 hours.
Remove the marshmallow from the pan. Dredge the entire marshmallow square in confectioners' sugar. With scissors, cut into 12 even marshmallows and then dredge each marshmallow again in confectioners' sugar. Store in an airtight container until ready to use.
For the s'mores bar: Arrange the components of the s'mores bar and then assemble s'mores as you desire. My favorite is marshmallows with bananas and peanut butter cups. They definitely have me saying, "S'more, please!"