BASE:

• ½ cup (1 stick) butter
• One 1-ounce square unsweetened chocolate
• 1 cup all purpose flour
• 1 cup granulated white sugar
• ¾ cup chopped walnuts
• 2 large eggs
• 1 teaspoon baking powder
• 1 teaspoon vanilla extract

FILLING:

• 6 ounces cream cheese, at room temperature
• ¼ cup (½ stick) butter, at room temperature
• ½ cup granulated white sugar
• 2 tablespoons all purpose flour
• ½ teaspoon vanilla extract
• 1 large egg
• ¼ cup chopped walnuts
• 6 ounces semisweet chocolate chips
• 2 cups mini marshmallows

FROSTING:

• ¼ cup (½ stick) butter
• 2 tablespoons milk
• One 1-ounce square unsweetened chocolate
• 2 ounces cream cheese
• 3 cups powdered sugar, sifted
• 1 teaspoon vanilla extract

• Preheat the oven to 350°F. Spray a 13x9-inch pan with baking spray.

MAKE THE BASE:

• In a large saucepan over low heat, melt the butter and chocolate, whisking constantly until well blended. Whisk in the flour, and then add the remaining base ingredients. Mix well. Spread into the prepared pan.

MAKE THE FILLING:

• In a small bowl, combine all of the filling ingredients excepts the nuts, chocolate chips and marshmallows. Beat 1 minute at medium speed until smooth and fluffy, then stir in the nuts. Spread the mixture over the base, and sprinkle evenly with the chocolate chips.
• Bake 25 to 30 minutes, or until a toothpick inserted into the center comes out clean. Remove the pan from the oven and immediately sprinkle marshmallows on top. Return to the oven and bake an additional 2 minutes.

MAKE THE FROSTING:

• In large saucepan over low heat, combine the butter, milk, chocolate and cream cheese, stirring until melted and blended. Remove from heat; stir in the powdered sugar and vanilla until smooth. Pour the frosting over the marshmallows and lightly swirl with a knife to marble.
• Refrigerate until firm. Cut into bars.