Preheat your oven to 350°F (175°C) and position a rack in the middle. Line an 8×8-inch square metal baking pan with foil, dull side up, leaving a couple of inches of overhang on two sides. This makes it easier to lift out the bars for slicing later.

In a medium bowl, combine 8 tablespoons unsalted butter at room temperature, 1/3 cup powdered sugar, and 1/2 teaspoon kosher salt. Using an electric handheld mixer, beat on low speed to mix, then increase to medium and continue beating until smooth and well combined, about 2-3 minutes. Scrape down the sides as needed, then beat in 1 1/2 teaspoons vanilla extract. Add 1 cup all-purpose flour and mix on low until a crumbly dough forms. Fold the dough with a spatula to ensure no dry spots. Scatter the dough evenly on the prepared pan bottom and press it flat using the back of a measuring cup or a small glass (use parchment or plastic wrap if it sticks). Chill in the fridge for 10 minutes, then prick the crust all over with a fork.

Bake the chilled crust for 15-20 minutes until the surface looks matte and edges begin to color. While still warm, gently press down on the crust with the back of a glass to compress it. Let cool on a wire rack while you prepare the filling. Keep the oven on for the next baking step.

Place 5 1/3 tablespoons unsalted butter in a light-colored saucepan over medium-low heat. Once melted, increase heat slightly and cook, stirring frequently, until the butter foams, turns clear gold, and starts to brown (5-7 minutes), watching carefully to avoid burning. Carefully pour in 1/2 cup maple syrup; reduce heat to low and stir until the syrup thickens and darkens slightly (about 2 minutes). Remove from heat, scrape into a large bowl and cool for 5-10 minutes. Whisk in 1/2 cup packed brown sugar, 1/2 teaspoon kosher salt, 1 1/2 teaspoons vanilla extract, 2 tablespoons heavy cream, 2 teaspoons apple cider vinegar, 2 large eggs, and 1 tablespoon all-purpose flour until smooth. Fold in 1 1/4 cups toasted, roughly chopped pecans.

Pour the maple pecan filling over the baked and cooled crust. Bake for about 25 minutes until the filling is browned and set with a slight jiggle in the center. Remove from oven and immediately sprinkle flaky salt over the top while hot. Allow bars to cool completely at room temperature. For clean slices, refrigerate before cutting.

Once cooled and chilled, use the foil overhang to lift bars from the pan. Cut into desired sizes with a serrated knife. Serve slightly chilled or at room temperature. Store leftovers in an airtight container up to 5 days or freeze for longer storage.