Preheat oven to 350°. Whisk together flour and next 5 ingredients in a medium bowl. Grease (with shortening) and flour a 13- x 9-inch pan. Beat eggs, sugar, and vanilla at medium-high speed with a heavy-duty electric stand mixer about 3 minutes or until mixture is very light brown and ribbons form when beater is lifted. Add mayonnaise, and beat at low speed until combined. Add flour mixture to egg mixture alternately with hot water, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Pour batter into prepared pan. Bake at 350° for 30 to 35 minutes or until a wooden pick inserted in center comes out clean. Cool completely on a wire rack (about 1 hour). Spread Chocolate-Cream Cheese Frosting on cake.