Line an 8x8 baking pan with parchment paper or aluminum foil and set aside. Mix together in a large bowl the ¼ cup of melted unsalted butter, graham cracker crumbs, powdered sugar, and Speculoos cookie butter until well combined and slightly firm. As this cookie bar is extremely moist, the mixture will be soft.
Press the mixture firmly into the baking pan. Tip-Use a measuring cup to pack it into the pan evenly.
Melt the semi-sweet chocolate chunks with the 3 tablespoons of unsalted butter in the microwave or stovetop. Stir until smooth. Spread evenly on top of the cookie butter mixture.
Chill in the refrigerator until firm for 1 hour. Let the bars sit out for 15 minutes before cutting. Use a long, sharp knife dipped in hot water to cut the bars. Clean the knife with a wet towel and dip in the hot water after each cut for an even, clean cut. Serve chilled or set out at room temperature for at least 10 minutes prior to serving. Enjoy!