When ready to serve, you can make the marshmallow fluff. (You can also add this prior to serving, you'll just need to torch it again to warm it up enough that it's softer. It firms when it sits).
Add 1/4 cup of water to the bowl of your stand mixer. Sprinkle the gelatin over top.
In a small saucepan over medium heat, add 1/4 cup water and 1/2 cup maple syrup, vanilla, and sea salt.
Bring the mixture to a boil, stirring frequently, until a candy thermometer reaches 220 degrees. This usually takes about 10 minutes.
Turn the stand mixer on low, letting the whisk attachment mix up the water and gelatin that had been sitting in it. With it still running, slowly pour in the maple syrup mixture.
Turn the speed up to medium-high and allow it to beat for 10 minutes or so, until you get a super fluffy marshmallow fluff consistency. Be careful not to mix it too long or it can start to firm up.
Immediately pour the marshmallow fluff over the chocolate ganache layer, spreading it out with a spatula.
Let the tart sit at room temp for 10 minutes or so, allowing the marshmallow layer to set up a bit. You can also throw it in the fridge for 5-10 minutes or so to speed up the process. Once it's pretty firm to touch, you can use a torch to toast it! Cut into slices and enjoy!
*NOTE ON STORING: I do store mine in the freezer. The marshmallow doesn't freeze through. Then when i'm ready to eat it, i'll let it thaw at room temp for 10-15 minutes and use my torch to heat up the sides and tops of the marshmallow to make it softer and fluffy again.