Prep the pan. Heat the oven to 350°F (175°C).
Line a 9x13-inch baking pan with parchment, leaving overhang for easy lifting. Grease lightly.
Melt the butter. In a large bowl, whisk the melted, slightly cooled butter with the brown sugar until glossy and well combined.
Add eggs and vanilla. Whisk in the eggs, one at a time, until the mixture looks smooth and thick. Stir in the vanilla.
Mix dry ingredients. In a separate bowl, whisk the flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
Combine wet and dry. Add the dry ingredients to the wet mixture.
Stir with a spatula just until no dry streaks remain. Do not overmix.
Fold in apples. Gently fold in the chopped apples (and nuts, if using). The batter will be thick—this is right.
Spread and swirl. Scrape the batter into the pan and smooth the top.
Warm the caramel slightly if it’s very thick. Dollop spoonfuls over the surface and swirl lightly with a knife. Do not over-swirl—you want ribbons, not complete coverage.
Bake. Bake for 25–30 minutes, until the edges are set and golden and the center springs back lightly. A toothpick inserted into the center should come out with a few moist crumbs, not wet batter.
Cool and slice. Let the blondies cool in the pan for at least 30–45 minutes.
Lift out using the parchment and cut into bars. For clean slices, chill for 20 minutes before cutting.
Finish (optional). Drizzle with a little extra caramel and a pinch of flaky sea salt before serving.