To flatten the beer, open the bottle and let it sit overnight; or, stir the beer with a whisk to release as much of the carbon dioxide as possible.
Spray one standard bread loaf pan with nonstick spray.
For the bloom, sprinkle the gelatin in the bowl of a stand mixer. Mix the vanilla and flattened beer, and pour this over the gelatin. Whisk until no lumps remain. Set the bowl back into your mixer and fit the mixer with a whisk attachment.
For the sugar syrup, combine the flattened beer, corn syrup, sugar, and salt in a 4-quart saucepan or larger. Clip a candy thermometer to the side. Turn the heat to medium-high and bring the sugar mixture to a boil. As the syrup heats, it will foam up to nearly fill the pan. Keep an eye on it so it doesn’t boil over. When the mixture is between 225° and 230°, let it bubble for another 5 minutes and then remove it from heat.
With the mixer on low speed, carefully pour the sugar syrup down the side of the bowl into the gelatin bloom. Turn the mixer to high once all the syrup has been added and let it whip for 8-10 minutes, until it looks like glossy meringue and is very thick. Pour the marshmallow into the loaf pan and let it cure, uncovered, for 10-12 hours or overnight.
When the marshmallows are cured, rub the top with a little powdered sugar and turn the marshmallows out onto a cutting board. Rub the top with more powdered sugar. Cut into squares of equal size.