Preheat the oven to 350°F. Line a 13x9 inch metal baking pan with parchment paper.
Whisk flour, baking powder, and salt in a medium bowl and set aside. Melt butter in a medium skillet over medium heat, swirling frequently, until browned bits cling to the bottom and the butter smells nutty. Transfer to a medium bowl.
Beat brown sugar into the brown butter until the mixture resembles wet sand, 2 to 3 minutes. Add eggs and vanilla and beat until fluffy, about 2 minutes. Add dry ingredients and fold until combined.
Spread batter evenly in the prepared pan using an offset spatula. Bake until golden brown and a tester inserted in the center comes out with a few moist crumbs, 20 to 25 minutes. Cool completely in the pan on a wire rack.
Toast pecans in the 350°F oven, tossing every few minutes, until a nutty aroma fills your kitchen, 10 to 12 minutes. Watch them carefully as they can burn quickly.
Pour maple syrup into a deep, heavy-bottomed pot. Bring to a boil over medium heat and boil gently, stirring every few minutes, until the syrup coats the back of a spatula and cools quickly to a caramel-like consistency, about 15 minutes.
Stir in the butter until fully melted, then add the cream, stirring constantly. Add the sea salt and stir to combine. Remove from heat and fold in the toasted pecans.
Pour the caramel over the cooled blondie and spread evenly. Refrigerate for 1 hour, then lift from the pan, cut into bars, and sprinkle with fleur de sel.
Notes
Yield: 24 bars from a 13x9 inch pan, each approximately 2x2 inches. Doneness Cue: The blondie is ready when the top is golden brown and a tester inserted in the center comes out with a few moist crumbs attached, not wet batter. Storage: Store bars in an airtight container in the refrigerator for up to one week. Let them come to room temperature before serving for the best texture. Make Ahead: The finished bars can be made up to one week in advance and stored in an airtight container in the refrigerator.