01 - Line a 9x9-inch baking dish with parchment paper and lightly grease the surface 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, approximately 4-5 minutes. Remove from heat and allow to cool slightly.
03 - In a large saucepan, melt 1/4 cup butter over low heat. Add mini marshmallows and stir continuously until completely melted and smooth. Remove from heat and incorporate vanilla extract.
04 - Quickly fold the cooked apple mixture into the marshmallow base, then add the rice cereal. Stir until all components are evenly distributed and coated.
05 - Transfer the mixture to the prepared baking dish. Press firmly and evenly across the pan, smoothing the top surface with a spatula.
06 - If desired, drizzle melted white chocolate over the top surface while still slightly warm for decorative finish.
07 - Allow the treats to cool at room temperature for at least 1 hour until completely set before cutting into 16 squares.