Start by preheating your oven to 350°F (175°C). Then, grease and flour a 15x10x1-inch jelly roll pan to ensure the bars don't stick during baking.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until the mixture is light and fluffy. This step ensures a smooth and creamy texture for the bars.

Add the flour, baking powder, salt, milk, vanilla extract, and eggs to the creamed butter and sugars. Mix everything together at medium speed until a smooth batter forms. This combination of ingredients creates the base for your bars.

Gently fold in the chopped maraschino cherries, chocolate chips, and chopped nuts into the batter. This adds delightful bursts of flavor and texture to the bars, making each bite a treasure trove of deliciousness.

Transfer the batter into the prepared jelly roll pan, spreading it out evenly with a spatula. Place the pan in the preheated oven and bake for 25 to 30 minutes or until the bars are golden brown and a toothpick inserted into the center comes out clean.

While the bars are baking, prepare the icing by combining powdered sugar, softened butter, milk, and vanilla extract in a small bowl. Mix until smooth and creamy, ready to be spread over the warm bars.

Once the bars are done baking, remove them from the oven and immediately spread the prepared icing evenly over the warm bars. Then, melt the unsweetened chocolate square and drizzle it over the icing in decorative patterns for an added touch of indulgence.

Allow the bars to cool completely in the pan before cutting them into squares or rectangles. Once cooled, these delightful Treasure Chest Bars are ready to be enjoyed as a sweet treat for any occasion.