Let’s transform those ingredients into a delectable dessert:

1. Preheat your oven to 350°F (175°C) and line a 9×9-inch baking pan with parchment paper, leaving a bit of overhang for easy removal.
2. In a large mixing bowl, combine the melted butter and sugar. Whisk until smooth and creamy, about 2 minutes.
3. Add in the eggs and vanilla extract. Continue whisking until the mixture is well combined and slightly thickened.
4. In another bowl, sift together the flour, cocoa powder, baking powder, and salt. Gradually fold the dry ingredients into the wet ingredients until just combined.
5. Gently fold in the diced apples and nuts, if using, until evenly distributed throughout the batter.
6. Pour half of the brownie batter into the prepared baking pan. Drizzle 1/4 cup of caramel sauce over the top, then pour the remaining batter over the caramel.
7. Swirl the top with a knife to create a marbled effect, then drizzle additional caramel sauce for an extra touch of sweetness.
8. Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake!
9. Allow the bars to cool completely in the pan before lifting them out and slicing into squares.

Remember, these bars are meant to be enjoyed with a little bit of mess—sticky fingers and all!