Preheat your oven to 350 °F.
Butter or line an 8x8 inch baking pan.
In a large bowl, microwave your butter in 30-second bursts until completely melted. Mix in brown sugar and beat until completely combined.
Beat in eggs, vanilla, and salt until batter is smooth.
Using a wooden spoon or spatula (not a whisk) gently stir in flour until just combined, being careful not to overmix. Pour batter into the prepared baking pan.
With a spoon drop 4-5 dollops of caramel sauce evenly over the top of your batter. Using a butter knife, marble the caramel sauce into the batter by running the knife up and down the length of the pan in evenly spaced rows. Wipe any excess batter off your knife and repeat this process, this time moving left to right. I suggest doing this only once as any more marbling can cause the caramel to mix completely into the batter, leaving no distinct layers.
Bake for 20-25 minutes, or until golden brown and a toothpick inserted into the center comes out clean of raw batter. If caramel sauce sticks to the toothpick, that's okay! If you're unsure whether the blondies are baked through, give the pan a little shake and make sure the center is set and doesn't jiggle.
Let cool to room temperature before slicing and serving.