Grease a 9×13 baking pan with butter, sprinkle with powdered sugar until well coated, set aside.
In the bowl of a stand mixer add ½ cup cold flat chocolate stout beer. Sprinkle with gelatin. Allow to stand while the sugar is being prepared.
In a large saucepan over medium heat, add the remaining ½ cup beer, sugar and corn syrup. The mixture will bubble up considerably. Stir until the sugar has dissolved.
Raise heat to high and allow to boil until the mixture reads 240 degrees Fahrenheit on a candy thermometer (about six to eight minutes).
Once the temperature has been reached, turn off heat.
Turn the mixer on low and slowly pour the hot sugar mixture into the gelatin. Once all the sugar has been added, turn the mixer on high until the mixture is light and fluffy and tripled in volume, about six minutes.
While the sugar and gelatin are being mixed, prepare the egg whites. Add the egg whites to a bowl with the salt. Beat on high with a hand mixer until stiff peaks form.
Gently fold the egg whites and vanilla extract into the stand mixer ingredients until just combined.
Pour the marshmallows into the prepared pan. Sprinkle with powdered sugar. Allow to sit at room temperature until set, about two hours. Remove from pan, cut into squares.