In a medium bowl, stir together melted butter, tahini, and both sugars until well combined. Add in beaten egg. In a separate bowl, whisk together flour, baking soda, powder, and salt. Combine wet and dry mixtures, fold together (it will be slightly dry so you may knead with your hands). Fold in chocolate chips. Press firmly into a 9x9 baking pan and add more chocolate chips on top. Bake at 350 degrees for 22 minutes or until golden brown around the edges. Cool for 10 minutes before cutting!