Heat the oven to 350°F (175°C). Line the pan with a parchment sling. Choose a thick caramel sauce rather than a thin beverage syrup.
Whisk melted cooled butter with brown sugar. Add egg, extra yolk, and vanilla and whisk until the mixture looks smooth and glossy.
Whisk flour, baking powder, and fine salt in a separate bowl. Fold into the wet mixture until the last dry streak disappears.
Spread batter evenly in the pan, reaching the corners. Dollop caramel in small spoonfuls across the surface instead of pouring one large pool.
Drag a thin knife through the caramel in a few broad strokes. Keep the swirls near the top rather than stirring down to the base.
Bake 25–32 minutes, until the edges are golden and the blondie portion at the center has moist crumbs, without raw batter.
Scatter flaky salt over the warm surface. Cool completely in the pan on a rack so the caramel can settle and thicken.
Lift out and cut into 16 squares with a sharp knife. Wipe between cuts to prevent caramel from dragging through the tender crumb.
