The biggest trick to baking with caramel is to let the baked goods cool completely after baking. Wait, wait, wait. Two hours at minimum. Also, make sure you don't overbake these blondies. Dry blondies are no fun.
Preheat oven to 350 degrees (F). Butter a 9 x 9-inch baking pan or spray with nonstick cooking spray. Set aside.
In a small bowl, mix the flour, baking powder, and salt. Set aside.
In a large bowl, cream together butter and sugar until light and fluffy. Add in eggs, vanilla, and flour mixture. Mix until well combined.
Spread half of the mixture in the prepared pan and smooth.
Spread salted caramel onto the blondie batter using a spoon, leaving a bit of space around the edges. Top with the chopped nuts. Drop the remaining batter in spoonfuls over the caramel and spread with a spatula. Using a knife, swirl the blondie batter with the caramel. If desired, sprinkle with fleur de sel.
Bake 28 - 32 minutes until set in the middle and golden brown. A toothpick should come out clean. Do not overbake.
Make sure you cool completely, at least 2 hours, before cutting into squares. If necessary, place in the refrigerator for 30 minutes to help the caramel set.
In a 3-quart saucepan, add the sugar and water and cook at medium heat. Stir until combined. Do NOT stir again. Heat the mixture until a candy thermometer reads 350 degrees (F). Remove thermometer.
Carefully add the butter and whisk until butter is melted. Remove the pan from heat and slowly pour in the cream, whisking all the time until the cream is incorporated. Whisk in the sea salt and set aside to cool.
Allow to cool for about 5 minutes, then store in a heatproof, airtight container until ready to use. Refrigerate leftovers.