Preheat your oven to 350°F (175°C). It’s important to get your oven ready because a consistent temperature is key to baking these fudge bars evenly. Meanwhile, grab your 13x9-inch pan and spray it lightly with baking spray to prevent sticking.
In a large saucepan over low heat, melt the butter and unsweetened chocolate, whisking constantly. This should take a few minutes, and you’ll want to ensure that the mixture is smooth and well blended before moving on.
Once melted, remove the pan from the heat and whisk in the all-purpose flour. This will help to thicken the mixture and create a batter-like consistency.
Add in the granulated white sugar, followed by the chopped walnuts, eggs, baking powder, and vanilla extract. Mix everything until fully combined. You'll notice a rich, thick batter forming, which is exactly what you want.
Spread the batter evenly into your prepared pan. Use a spatula to smooth it out, making sure it reaches the corners for even baking.
In a separate small bowl, combine the filling ingredients: cream cheese, butter, granulated sugar, flour, vanilla extract, and egg. Beat this mixture at medium speed for about one minute until it’s smooth and fluffy.
Stir in the remaining walnuts and mix well. Now, carefully spread this filling mixture over the fudge base in the pan, ensuring it’s evenly distributed.
Sprinkle the semisweet chocolate chips on top of the filling, allowing those little pockets of chocolate to melt into the mixture as it bakes.
Place the pan in the oven and bake for 25 to 30 minutes. Check for doneness by inserting a toothpick into the center—if it comes out clean, your fudge bars are ready!
As soon as you remove the pan from the oven, top it off immediately with the mini marshmallows. They’ll melt slightly from the heat, creating a gooey topping.
Return the pan to the oven and bake for an additional 2 minutes, just to soften the marshmallows.
In a large saucepan over low heat, combine the remaining butter, milk, unsweetened chocolate, and cream cheese. Stir until melted and blended, then remove from heat.
Stir in the powdered sugar and vanilla extract until smooth. This mixture will form a lovely frosting.
Pour the frosting over the marshmallows, and take a knife to lightly swirl it in for a marbled effect. This adds an extra layer of deliciousness.
Finally, refrigerate your Rocky Road Fudge Bars until they are firm. Once set, cut them into bars and enjoy!