1.Remove All String From The Roast And Trim Away Excess Fat. Place The Chili Powder In A Bowl; Blend In The Oil, Chili, Pepper, Cumin, Coriander, Paprika, Allspice, Salt And Garlic To Make A Paste. Rub All Over The Pork, Working The Spice Mixture Into The Meat Well. Marinate For At Least 30 Minutes Or Overnight. Transfer The Roast To A Slow Cooker. 2.Blend The Ketchup With The Vinegar, Molasses And Mustard. Pour Mixture Over The Roast And Cook On Low For 8 To 10 Hours Or Until Very Tender. 3.Transfer The Roast To A Large Bowl; Discard Any Visible Fat. Use Two Forks To Separate The Meat Into Long Strands. Remove 1 1/2 Cups Of The Cooking Juices And Strain Into A Saucepan. Whisk In The Cornstarch And Bring To A Boil. Cook, Stirring, Until Thickened And Bubbling. Return The Shredded Meat To The Slow Cooker; Stir To Combine With Remaining Cooking Juices. Serve The Meat On Soft Rolls; Drizzle With Thickened Sauce Mixture To Taste.