Preheat oven to 350°F. Grease (with shortening) and lightly flour a 13- x 9-inch pan. Mix flour, baking powder, baking soda, and salt in a medium bowl; set aside.

Beat brown sugar and butter at medium speed with an electric mixer until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Beat in vanilla. Alternately add flour mixture and buttermilkto butter mixture, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Pour batter into prepared pan.

Bake at 350°F for 32 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack 10 to 15 minutes; remove from pan to wire rack, and cool completely (about 1 hour).

Make the Simple Rum Frosting: Beat 3 cups powdered sugar and 3/4 cup softened butter at medium speed with an electric mixer until light and fluffy. Beat in 2 Tbsp. rum. Spread top of cake with frosting; sprinkle with pecans.