Double Chocolate Mud Bars

1/2 cup butter, softened
1 cup granulated sugar
2 large eggs, separated
1 1/2 cups all-purpose flour
1 teaspoon baking powder
1/2 teaspoon salt
1 cup chopped walnuts
1/2 cup semisweet chocolate chips
1 cup miniature marshmallows
1 cup firmly packed brown sugar

Preheat oven to 350 degrees F. Grease a 13 x 9-inch baking pan.

Beat together butter and sugar at a medium speed until light and fluffy. Beat in egg yolks, one at a time, until blended.

Mix together flour, baking powder, and salt. Fold flour mixture into butter mixture. Press mixture into prepared pan. Pack down firmly. Sprinkle nuts, chocolate chips, and marshmallows over the top of mixture pan.

Beat egg whites at high speed until stiff peaks form. Fold in brown sugar. Spread over mixture in pan. Bake for 35 minutes or until a crust is formed.

Transfer pan to a wire rack to cool completely. Cut into squares and serve.