Prepare the Pan: Spray a 9×9-inch pan with non-stick spray and line it with a foil or parchment paper sling. Spray the lining again to ensure easy removal.
Brown the Butter: Place a large pot on the stove over medium heat and add the butter. Allow the butter to melt and then continue cooking until it turns a golden brown color and develops a nutty aroma. Be sure to stir occasionally to prevent burning. Once browned, remove the pot from the heat.
Melt the Marshmallows: Immediately add the sea salt and 8 cups of mini marshmallows to the pot with the browned butter. Stir until the marshmallows are completely melted and the mixture is smooth. If necessary, return the pot to low heat to finish melting the marshmallows, but be careful not to burn the butter.
Add the Vanilla: Once the marshmallow mixture is smooth, stir in the vanilla extract.
Mix in the Cereal: Measure out the Rice Krispies cereal in a large bowl. Pour the melted marshmallow mixture over the cereal and mix well until evenly coated.
Incorporate Remaining Marshmallows: While the mixture is still warm, gently fold in the remaining 2 cups of mini marshmallows.
Layer the Treats: Lightly press about half of the Rice Krispies mixture into your prepared pan, flattening it down so it’s even. Spread the dulce de leche evenly over this layer.
Finish and Set: Top with the remaining Rice Krispies mixture, lightly pressing it down to create an even layer. Let the treats set at room temperature until fully cooled and firm.
Serve: Once set, use the parchment or foil sling to lift the treats out of the pan. Cut into squares and serve.