Special equipment: a candy thermometer; 20 craft sticks or short, thick skewers
Fill a glass with ice water and set aside. Line a cookie sheet with waxed paper and sprinkle it generously with confectioners' sugar; set aside.
Combine the brown sugar, sorghum and butter in a saucepan over medium. Heat until the butter and sugar melt, 3 to 5 minutes. Add the condensed milk and bring to a simmer; cook for about 1 minute. Lower the heat and continue to cook, stirring constantly, until the mixture registers 240 degrees F on a candy thermometer or until a soft ball forms when a spoonful of the caramel is dropped into the glass of ice water, 15 to 20 minutes. Let stand for 5 minutes.
Skewer the marshmallows and dip them in the caramel so that the sides are covered but the tops are not. Place on the prepared cookie sheet to cool. Before the caramel sets, sprinkle the dipped sides with a little flaky sea salt.
Serve immediately or wrap each candy in waxed paper to enjoy at a later time--if it's really humid where you are, they won't hold up for long!