Step 1: Preheat your oven to 350°F (175°C) and grease an 8×8 inch baking pan or line it with parchment paper.
Step 2: In a large bowl, combine the melted butter, granulated sugar, and brown sugar. Whisk until well combined.
Step 3: Add the eggs and vanilla extract, whisking until the batter is smooth and slightly lightened in color.
Step 4: Sift in the flour, cocoa powder, and salt. Fold gently with a spatula until just combined.
Step 5: Spread half of the batter into the prepared pan. Evenly distribute the diced apples over the batter.
Step 6: Drizzle half of the caramel sauce over the apples. Top with the remaining brownie batter and smooth the surface.
Step 7: Drizzle the remaining caramel sauce over the top and use a knife to swirl it into the batter. Sprinkle with flaky sea salt.
Step 8: Bake for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let cool completely before slicing.