How to Make It
Preheat oven to 350°F. Stir together first 5 ingredients in a large bowl; make a well in center of mixture. Whisk together buttermilk and next 5 ingredients; add to flour mixture, stirring just until dry ingredients are moistened. Pour batter into a lightly greased (with shortening) and floured 13- x 9-inch pan.
Bake at 350°F for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 minutes; remove from pan to wire rack, and cool completely (about 1 hour).
Make the Mascarpone Frosting: Beat 1 (3-oz.) package softened cream cheese and 1/4 cup softened butter at medium speed with an electric mixer until creamy; gradually add 5 1/3 cups powdered sugar, beating at low speed until blended after each addition. Add 1 (8-oz.) package mascarpone cheese and 2 tsp. vanilla extract, beating until blended.
Cut cake into 6 rounds using a 3 1/2-inch round cutter. Reserve remaining cake trimmings for another use. Split each mini cake in half horizontally. Spread about 1/3 cup Mascarpone Frosting between layers; spread remaining frosting on tops of cakes. Store cakes in refrigerator until ready to serve.