Preheat oven to 350°F.
Melt butter in 8- or 9-inch skillet over medium heat.
Stir in brown sugar. When thoroughly blended, stir in berries. Cook, stirring, until mixture bubbles and berries begin to give up juice. Add 1/2 cup white sugar and mash berries slightly. Cook 5 minutes more, then remove from heat. Cream shortening and remaining 1 cup white sugar until light and fluffy. Beat in eggs, one at a time. Sift together dry ingredients and add alternately with combined milk and vanilla to creamed mixture. Blend well, but do not overbeat. Pour batter over blackberry mixture in skillet. Bake 35 to 40 minutes until center of cake springs back when touched lightly. Cool in pan until lukewarm, then turn out onto large serving plate. Serve topped with cream.