For the Rum Raisin Sauce, place all ingredients in blender container; cover. Blend on high speed until smooth. Pour into medium saucepan. Cook on medium heat 10 to 12 minutes or until thickened, stirring frequently. Set aside.
Preheat oven to 425°F. For the Bars, place 1/2 cup of the oats in food processor. Pulse to coarsely chop. Add flour, brown sugar and 1 teaspoon of the cinnamon; pulse to mix well. Add cold butter; pulse until mixture resembles coarse crumbs. Add 2 tablespoons of the buttermilk; pulse to mix well. Reserve 1/4 cup in small bowl. Press remaining oat mixture into bottom of greased foil-lined 9-inch square baking pan. Mix reserved oat mixture and remaining 1/4 cup oats. Spread on small baking sheet.
Bake crust and oat crumble together in oven 8 to 12 minutes or until golden brown. Remove crust and oat crumble from oven. Reduce oven temperature to 325°F. Reserve oat crumble for serving. Pour Rum Raisin Sauce into prepared crust.
Meanwhile, mix granulated sugar and melted butter in large bowl with wire whisk until well blended. Add eggs, mix well. Stir in 1/2 cup of the remaining buttermilk, remaining 1/2 teaspoon cinnamon and vanilla. Pour over sauce in crust.
Bake 45 to 50 minutes or until custard is set and just golden brown around the edges. Cool completely on wire rack.
Just before serving, mix confectioners’ sugar and remaining 2 tablespoons buttermilk until smooth. Sprinkle top of cooled dessert with oat crumble then drizzle with glaze. Cut into bars.