Spray a 10 x 15-inch baking sheet with nonstick cooking spray and set aside. Melt the butter in a large heavy-bottomed stainless steel pan over low heat. Add the sugars, water, molasses, allspice, and salt, stirring with a wooden spoon to combine and dissolve sugars. Increase to medium heat and attach a candy thermometer. Let the mixture boil, stirring constantly, until thermometer registers 300ºF, about 15 minutes. Remove from heat and stir in 1½ cups of the pecans. Pour the toffee onto the prepared sheet, spreading all the way to the edges. Sprinkle chocolate chips over the top and allow toffee to harden for one minute. Spread the chocolate over the toffee until completely melted. Sprinkle the remaining pecans over the top and refrigerate until completely chilled, about two hours. Break into bark pieces and transfer to airtight container. Toffee will keep tightly covered and refrigerated for two weeks.
