Heat oven to 375°F. Grease bottom and sides of 13x9-inch pan with shortening or cooking spray (do not use dark pan).
In medium bowl, beat 1 cup butter and 1 cup brown sugar with electric mixer on low speed until creamy. Stir in flour. Press evenly in pan.
Bake 12 to 14 minutes or until edges are golden brown and center springs back when touched lightly.
Meanwhile, in medium bowl, mix all filling ingredients except pecans. Stir in pecans. Pour over crust, spreading pecans evenly.
Bake 12 to 15 minutes or until filling is set. Cool completely, about 1 hour.
In small bowl, mix icing ingredients (add additional rum or water, 1/2 teaspoon at a time, if icing is too thick to drizzle). Drizzle icing over bars. For bars, cut into 8 rows by 4 rows.