Using a standing mixer fitted with the whisk attachment, combine the gelatin and 1/2 cup plus 1 tablespoon cold water; let stand for 30 minutes.
Meanwhile, in a small, heavy saucepan, combine the granulated sugar, corn syrup, salt and 1/2 cup water and cook over low heat, stirring, until the sugar dissolves. Increase the heat to high and cook the syrup until it registers 245° on a candy thermometer, about 12 minutes. Remove from the heat.
With the standing mixer on low speed, slowly pour the hot syrup into the softened gelatin. Increase the speed to high and beat until the mixture is very thick, white and has almost tripled in volume, about 12 minutes. Beat in the vanilla.
Meanwhile, dust an 8-by-12-inch glass baking dish with
confectioners' sugar. Scrape the marshmallow mixture into the pan and sprinkle with more confectioners' sugar. Refrigerate, uncovered, overnight. Cut into 1-inch squares.
Place the peanut butter and the butter in a glass measuring cup and microwave until melted; stir to combine. Let cool.
Place a cooling rack on a baking sheet. Using a fork, dip each marshmallow into the peanut butter mixture and transfer to the rack.
Place the nuts in individual shallow bowls. Roll the
marshmallows in the nuts. Transfer to a plate and refrigerate until set.