Preheat your oven to 350°F (175°C). Line a 9x13 inch baking dish with parchment paper, leaving an overhang on the sides for easy removal.
In a medium bowl, combine diced apples with the sugar, cinnamon, and nutmeg. Toss until the apples are evenly coated. Aim for ¼-inch apple pieces for the perfect texture balance.
In a large bowl, whisk together the melted butter and brown sugar until smooth and the sugar begins to dissolve—about 2 minutes of vigorous whisking.
Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, salt, cinnamon, and nutmeg.
Gradually add the dry ingredients to the wet mixture, folding gently with a spatula until just combined. Stop when you no longer see streaks of flour to avoid over-mixing.
Spread about two-thirds of the blondie batter into your prepared baking dish. Distribute the apple mixture evenly over the batter.
Dollop the remaining batter over the apples—it doesn't need to cover completely as this creates caramel pockets.
Bake for 35-40 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Allow the blondies to cool in the pan for about 15 minutes. While still warm (but not hot), drizzle the caramel sauce evenly over the top.
If using, sprinkle with flaky sea salt to create a sweet-salty contrast.
Cool completely before cutting—for the sharpest cuts, refrigerate for 30 minutes after cooling, then use a warm knife.