Step 1: Grease a 9x9 inch baking pan or line it with parchment paper for easy removal.
Step 2: In a large pot, melt 4 tbsp butter over low heat. Add marshmallows and stir until completely melted. Remove from heat and fold in the puffed rice cereal until evenly coated.
Step 3: Gently press the cereal mixture into the prepared pan. Do not pack too tightly. Let it cool.
Step 4: In a medium saucepan, combine the 1/2 cup butter, brown sugar, and sweetened condensed milk. Cook over medium-low heat, stirring constantly until the mixture thickens and turns a light caramel color (about 5-7 minutes).
Step 5: Pour the caramel mixture over the rice krispie base and spread evenly. Allow to cool completely at room temperature or in the fridge for 20 minutes.
Step 6: Melt the chocolate chips and heavy cream together in a microwave-safe bowl in 30-second increments, stirring until smooth.
Step 7: Spread the chocolate ganache over the cooled caramel layer. Sprinkle with sea salt if desired.
Step 8: Refrigerate for at least 1 hour before slicing into squares.