2 tablespoons unsalted butter, plus more for the dish
1 1/4 cups packed dark brown sugar
1/4 cup light corn syrup
1 cup heavy cream
1 1/2 cups coarsely crushed thin pretzels
Pour the beer into a glass; let sit until flat, about 1 hour.
Butter an 8-inch-square baking dish. Bring half of the beer to a boil in a small saucepan over medium heat; cook until reduced to 2 teaspoons, about 10 minutes.
Meanwhile, combine the remaining beer, the brown sugar, corn syrup, cream and butter in a large saucepan. Bring to a boil over medium-high heat and cook, stirring occasionally, until a candy thermometer registers 235 degrees F, 10 to 12 minutes. Remove from the heat and stir in the beer syrup and pretzels. Spread in the prepared dish and chill until firm, 45 minutes to 1 hour. Cut into pieces.