Preheat oven to 350°F. Lightly grease 13x9-inch baking dish.
Melt the butter in a large microwave-safe bowl (just until melted). Remove from microwave and stir in 2 cups of brown sugar. Mix well, then add eggs, flour, baking powder and vanilla. Mix just until combined. Fold in the apples.
Spread the batter into prepared dish. Chop cream cheese into small bits and mix with nuts, cinnamon and remaining 1/4 cup of brown sugar. Sprinkle evenly over the batter.