Position a rack in the center of the oven and heat it to 175°C. Lightly grease a 23 x 33 cm light-colored metal baking pan, then line the bottom and two long sides with parchment, leaving enough overhang to lift the slab later. Set the pan on a heat-safe work surface. Measure every ingredient before starting, since the caramel moves quickly once it reaches the correct amber color.

Combine the granulated sugar and water in a small heavy-bottomed saucepan. Cook over medium heat without stirring, swirling the pan gently only if the color develops unevenly. After 7 to 10 minutes, the syrup should be a clear medium amber and smell toasted, not burned. Remove it from the heat and carefully whisk in the warm cream, followed by the 42 grams butter and 3 grams salt. Cool for 15 minutes, until thickened but still pourable.

Place the melted, slightly cooled butter and brown sugar in a large mixing bowl. Whisk for about 45 seconds, breaking up every sugar lump; the mixture should look glossy and cohesive but not aerated. Add the eggs one at a time, whisking until each disappears before adding the next, then mix in the vanilla. Stop when the mixture falls from the whisk in thick ribbons. Vigorous extended whisking would introduce unnecessary air and encourage a cakier crumb.

In a medium bowl, whisk the flour, baking powder, and 3 grams fine sea salt until evenly distributed. Add the dry mixture to the wet ingredients in two additions, folding with a flexible spatula and scraping the bottom and sides after each addition. When only a few flour streaks remain, fold in the white chocolate. Stop as soon as the batter is uniform. It should be thick, glossy, and spreadable, with no pockets of dry flour.

Spread half the batter evenly in the prepared pan, pushing it into the corners with the spatula. Spoon about two-thirds of the caramel over the surface, keeping most of it 1 cm away from the pan edges. Cover with the remaining batter, then dot with half the reserved caramel and make four or five shallow swirls. Bake for 27 to 32 minutes. The edges should be golden and set, while a tester in the center should show moist crumbs but no fluid batter.

Set the pan on a wire rack and cool the blondies completely for 90 minutes without cutting them; the center continues to set as the sugar, butter, and chocolate cool. Refrigerate the pan for 30 minutes, then lift out the slab using the parchment. Warm the remaining caramel only until spoonable and drizzle it over the top, then add the flaky salt. Cut into 24 bars with a long sharp knife, wiping the blade clean with a warm damp cloth after every cut.