Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan or line it with parchment paper for easy removal.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
In a large bowl, combine the melted butter, brown sugar, and granulated sugar. Mix until well combined.
Add the eggs and vanilla extract to the butter mixture and stir until smooth.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.
Fold in the diced apples and nuts (if using) until evenly distributed.
Pour half of the batter into the prepared baking pan and spread it evenly. Drizzle half of the caramel sauce over the batter.
Pour the remaining batter over the caramel layer and spread it gently. Drizzle the rest of the caramel sauce on top.
Use a knife or toothpick to swirl the caramel into the batter for a marbled effect.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the blondies to cool in the pan for at least 15 minutes before cutting into squares.