In the bowl of a stand mixer, combine gelatin and 1/2 cup cold water. Set aside.
In a medium saucepan, combine 1/2 cup water, sugar, agave nectar or corn syrup, and salt.
Heat saucepan over medium high heat, stirring occasionally. Cook until mixture reaches 240 degrees F (about 7-10 minutes). Bubbles will be large and pop more slowly at this point
With mixer fitted with whisk attachment, gradually drizzle in sugar syrup, mixing on low speed until all syrup has been poured in.
Add dissolved coffee and vanilla extract to mixer and mix on speed 4 for 10-12 minutes or until mixture thickens and becomes glossy.
While mixer is running, prepare pan. Grease a 9×13″ pan and dust with powdered sugar.
Scrape marshmallow mixture into prepared pan and allow to rest at least two hours before cutting.
To cut marshmallows, turn onto cutting board dusted with powdered sugar. Grease a pizza cutter (or cookie cutter) and cut marshmallows into desired shapes. Roll each marshmallow in powdered sugar to coat.
Store finished marshmallows in airtight container for up to two weeks.