Grease an 8x8 inch baking pan with butter or non-stick spray.
In a large pot, melt the butter over medium-low heat. Continue cooking the butter, stirring frequently, until it turns a golden brown color and has a nutty aroma. Be careful not to burn it.
2/3 cup unsalted butter
Add the 5 cups mini marshmallows to the pot with the brown butter. Stir constantly until the marshmallows are completely melted and smooth. Remove the pot from the heat and stir in the vanilla extract and salt.
5 & 1/2 cups mini marshmallows, 1 teaspoon vanilla extract, 1/4 teaspoon salt
Add the chocolate crispy rice cereal to the marshmallow mixture and stir until well-coated.
6 cups chocolate crispy rice cereal
Gently fold in the chocolate chips and the rest of the mini marshmallows while the mixture is still warm, so they melt slightly and create a marbled effect.
1/3 cup semi-sweet chocolate chips
Press the mixture into the prepared baking pan using a spatula or wax paper. Press firmly to compact the mixture. Top with more chocolate chips and a sprinkle of sea salt then allow the treats to cool and set at room temperature for about 30 minutes before slicing into squares.