1Lightly mist a 9-by-13-inch baking pan with cooking spray. Line the pan with a 16-inch length of foil, positioning it perpendicular to the pan's length. Fold the foil widthwise so it fits neatly in the bottom of the pan, allowing the excess to overhang the sides. Mist the foil with cooking spray.
2Whisk together the flour, sugar, and salt in a large bowl, breaking up any sugar lumps. Drizzle in the melted butter and stir with a silicone spatula until the mixture is evenly moistened and crumbly. Transfer the mixture to the prepared pan. Using your hands, press the dough into an even layer on the bottom of the pan and about 1 inch up the sides (1/4 inch thick). Use a fork to poke holes in the bottom of the crust. Refrigerate, uncovered, until firm.
3Preheat the oven to 350°F with a rack in the middle position. Bake the chilled crust until lightly browned.
4Melt the butter in a 10-inch skillet over medium-high heat. Cook, swirling frequently, until the milk solids turn golden brown and the butter develops a nutty aroma. Pour the browned butter into a large heatproof bowl and let cool until just warm.
5Whisk white sugar, honey, cornmeal, and table salt into the browned butter. Add the whole eggs and egg yolk, whisking gently. Stir in the Lyle's Golden Syrup, heavy cream, vanilla extract, and cider vinegar until smooth. Pour into the crust and bake until edges are puffed and center has a slight wobble.
6Transfer the pan to a wire rack and sprinkle with flaky sea salt. Cool completely for at least 3 hours. Use the foil overhang to lift bars out, then cut into 16 to 20 pieces.