Remove ham from fridge 1 hour before cooking. Preheat oven to 275 degrees F. Place ham in roasting pan flat side down. Add 1 cup water to bottom of pan. Cover pan with foil.

Transfer to oven and bake approximately 15 minutes per pound (2 hours total for an 8 pound ham) or until heated through.

Meanwhile, in medium saucepot, stir together pineapple, brown sugar, vinegar, mustard, soy sauce and 1/2 cup water. Heat to boiling over medium-high heat, then reduce heat to medium-low and simmer 10 minutes, stirring occasionally.

In small bowl, stir together cornstarch and 2 tablespoons water. Stir cornstarch mixture into pineapple mixture and increase heat to medium. Cook, stirring constantly, just until sauce thickens. Remove from heat and stir in butter and salt. Let sauce stand at room temperature until ham is ready.

Remove ham from oven, turn so the sliced side is facing up, and immediately brush with about half of pineapple sauce, getting some sauce between the ham slices if possible. Replace foil and let ham stand 20 minutes before serving.

Slice ham and serve with remaining pineapple sauce.