Preheat the oven to 350 degrees F. Line a 9x13 inch baking pan with parchment paper, allowing 2 sides to hang over the edges. Lightly grease the top of the paper and sides of the pan (I use baking spray with flour).
In a stand mixer fitted the the paddle attachment, cream the butter on medium-high speed for several minutes until light and fluffy. Turn the mixer off, add both sugars and the vanilla extract, and turn the mixer on low to combine everything. Turn the speed back up to medium-high for an additional 3 minutes. Scrape down the sides of the bowl.
With the mixer on low speed, add the eggs one at a time, allowing each egg to fully incorporate before adding the next, scraping down the sides after the 2nd and final egg.
With the mixer on low speed, add the tahini and cashew butter until evenly combined. Scrape down the sides.
With the mixer on low speed, add the flour in 3 stages until fully incorporated. Scrape down the sides of the bowl as needed, all the way to the bottom. If using, add the chopped cashews.
Using a spatula, scrape the batter into the prepared pan. It will be thick and almost sticky. Spread it evenly using a regular of offset spatula.
Drizzle the melted chocolate over the batter (see notes for melting options). Sprinkle the salt evenly over the top of the bars.
Bake for 30 minutes, or until a toothpick comes out of the center clean. Allow to cool completely, then lift the parchment overhangs to remove the bars from the pan. Slice into bars and serve.
Using crunchy cashew butter will add a touch of texture to the bars, but not much since it doesn’t account of a significant portion of the batter. If you’d like additional texture, add the chopped cashews.To quickly melt the chocolate, I recommend placing it in a microwave-safe liquid measuring cup. Microwave in 20-30 second intervals, stirring in between each interval, until just melted. You can also use a double boiler.