Prepare an 8x8 baking dish with parchment paper, leaving overhang on the edges.Place pitted dates in a big S-bladed food processor. Process on high for 1–2 minutes until dates become a thick paste.
Place date paste in baking dish. Spread paste evenly over dish bottom with damp fingertips. Use a spoon to evenly spread tahini over the date layer. Top with chopped walnuts equally.
Freeze for 2 hours or overnight to solidify the layers.Make a double boiler when the layers solidify. Simmer a few inches of water in a medium saucepan over medium-high heat. Put chocolate chips in a heat-safe dish.
Place the dish of chocolate chips on top of simmering water. The bowl bottom shouldn't touch the water. Stir and scrape the bowl as the chocolate melts to thoroughly melt it. Set aside the bowl after heating.
Remove the date-tahini layer from the freezer. Pull it out of the pan and place on a big cutting board with the parchment overhang on opposing sides. Remove parchment. Slice top-to-bottom with a sharp knife or bench scraper.
Place the bars on a parchment-lined baking sheet. Chocolate drizzle on top. Chocolate covering just part of each bar is fine.
Place the tray of chocolate-covered date bars in the freezer for 30 minutes to solidify. Uneaten bars may be refrigerated for a week in a sealed container. Enjoy!