Preheat the oven to 350 degrees F. Butter and flour a 9x13-inch baking pan.
Toast the cashews on an unoiled baking sheet for 7 to 10 minutes, golden; then chop, toss with 3/4 teaspoons of the salt, and set aside to cool.
In a heavy skillet or saucepan, heat the butter until it begins to bubble, turn amber colored, and leave tiny brown particles on the bottom of the pan. Strain through a fine-mesh sieve and refrigerate for 10 minutes.
Beat the brown sugar, egg and vanilla into the cooled butter until creamy. Sift in the remaining 1/4 teaspoons of salt and half of the flour; mix until blended. Sift in the rest of the flour and mix until the dough is stiff and forms a ball. Press the dough evenly into the bottom of the prepared pan and set aside.
For the syrup, warm the butter and sugar on medium-low heat in a saucepan just until the mixture bubbles vigorously and the sugar begins to melt. Add the water and stir until most of the sugar has melted into syrup. Toss the cashews in the syrup and spread evenly over the dough.
Bake for about 25 minutes, until just golden and firm. Cool and then cut into 24 bars--6 rows x 4 columns. Store in a container with a tight-fitting lid.