Set the oven to 325°F, fit parchment into an 8-inch square pan with 2 inches extending above two opposite sides, then use an electric mixer in a large bowl to beat the 12 tablespoons butter with the sugar on medium for about 2 minutes before incorporating the flour and first salt measurement on low. The butter and sugar should become fluffy before the flour goes in; stop the final mixing when the crumbs are uniform and resemble coarse, damp sand.

Pack the crumbs firmly across the prepared pan, level the surface, and bake for 30 to 35 minutes before placing the pan on a rack to cool. Look for a dry surface with a light golden-brown color rather than a deeply browned crust.

Combine the condensed milk, remaining butter, syrup, and remaining salt in a medium saucepan, then stir continuously with a heatproof spatula over medium heat for 10 to 20 minutes, adjusting the heat to prevent scorching; once cooked, stir in the vanilla and immediately distribute the caramel evenly over the baked crust. Before adding the vanilla, confirm that the thickened mixture has changed from pale milky yellow to a butterscotch-pudding shade and registers 220°F to 225°F on an instant-read thermometer.

Melt the chopped chocolate using short microwave bursts or a double boiler with frequent stirring, smooth it across the caramel, and chill for about 30 minutes; once set, loosen the two unlined edges with a sharp knife, lift the slab onto a cutting board by the parchment, and divide it into 32 small bars. The chocolate should form a set top before you release the slab from the pan.