Line a 9-inch square pan with parchment paper; grease paper. Bring sugar, water, and lemon juice to a boil in a small saucepan over medium-high heat. Boil 8 to 10 minutes or until sugar begins to brown. (Do not stir.) Stir in cream and 2 Tbsp. butter; remove from heat.
Let stand, stirring constantly, until no longer bubbling. Microwave sweetened condensed milk and semisweet chocolate morsels in a large microwave-safe bowl at HIGH 3 minutes, stirring at 1-minute intervals. Stir in rum, 2 1/2 Tbsp. butter, and salt. Immediately pour into prepared pan. Immediately pour caramel over chocolate mixture; gently swirl with a knife. Chill 2 to 4 hours.
Cut into pieces; wrap in wax paper. Store in refrigerator up to 1 week.
Let stand at room temperature 15 minutes before serving.
PACKAGE IN: Mini Gingham Cupcake Liners ($75 for 50; bakeitpretty.com)
