01 - Line a 9x9-inch baking dish with parchment paper and lightly grease it to prevent sticking.
02 - Melt 2 tablespoons butter in a small skillet over medium heat. Add diced apple, brown sugar, cinnamon, nutmeg, and salt. Cook, stirring frequently, until apples are tender and caramelized, about 4-5 minutes. Remove from heat and cool slightly.
03 - Melt 1/4 cup butter in a large saucepan over low heat. Add mini marshmallows and stir until completely melted and smooth. Remove from heat and stir in vanilla extract.
04 - Quickly fold the cooked apple mixture into the marshmallow mixture, then add rice cereal. Stir until everything is evenly coated and combined.
05 - Transfer mixture to the prepared baking dish. Press firmly and evenly with a spatula to compact the bars.
06 - If using, drizzle melted white chocolate over the top of the bars.
07 - Let cool at room temperature for at least 1 hour until completely set. Cut into 16 squares before serving.