Preheat oven to 350 degrees F. Line a 9-inch square baking pan with foil and spray with non-stick cooking spray; set aside.
In a medium bowl, whisk together the flour, baking powder and salt; set aside.
Place the melted butter in a large bowl and whisk in the brown sugar until completely dissolved. Add the eggs and vanilla extract and whisk to combine. Add the flour mixture and fold gently with a rubber spatula until all of the dry ingredients are completely incorporated.
Press half of the dough into the prepared pan, smoothing the top with an offset spatula. Pour the salted caramel sauce onto the center of the dough and spread into an even layer, leaving about a ½-inch of space around the edges. Drop the remaining cookie dough in spoonfuls over the caramel and gently spread the dough with an offset spatula until the caramel is covered. Sprinkle the bars with fleur de sel.
Bake the blondies for about 30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out with just a thin coating of caramel. Cool the bars on a wire rack to room temperature, then refrigerate for at least 30 minutes to allow the caramel layer to set. Cut into squares and serve. Store leftovers in an airtight container at room temperature for up to 4 days.