01 - Line a 9x9-inch baking dish with parchment paper and lightly grease it
02 - In a small skillet, melt 2 tablespoons butter over medium heat. Add diced apple, brown sugar, cinnamon, nutmeg, and a pinch of salt. Cook, stirring often, until the apples are tender and caramelized, about 4-5 minutes. Remove from heat and let cool slightly
03 - In a large saucepan, melt 1/4 cup butter over low heat. Add mini marshmallows and stir until completely melted. Remove from heat and stir in vanilla extract
04 - Quickly fold in the cooked apple mixture, then add the rice cereal. Stir until everything is evenly combined
05 - Press the mixture firmly into the prepared baking dish, smoothing the top with a spatula
06 - Drizzle melted white chocolate over the top if desired
07 - Let cool at room temperature for at least 1 hour before cutting into squares