Preheat the oven to 350°F / 177°C / gas mark 4. Get a 9x13" baking pan ready but do not grease it.
In the bowl of a stand mixer, combine the sugar and butter on medium speed until well-mixed. Pause to scrape down the sides. Add the salt and vanilla extract then continue mixing 1 more minute.
Using low speed, add the flour 1 cup at a time. Continue mixing until it looks like crumbs. Scoop one generous cup from the crumb mixture and set it aside.
Add the eggs to the stand mixer and beat on medium speed until the dough starts to clear the sides of the bowl. Scoop the dough into the baking pan and, using your fingers, gently press it down to form the cookie base. Bake 30 minutes or until lightly brown.
Remove the baking pan from the oven and scoop spoonfuls of the jam onto the cookie base. Use the back of a spoon to spread the jam evenly to the edges. Scatter the white chocolate over the top, then sprinkle the reserved crumble mixture over it all.
Bake an additional 25-30 minutes until the crumb topping turns light golden brown. Cool completely in the pan before cutting into bars.