Step 1: Place rack in center of oven and preheat the oven to 350 F. Generously grease a 13-by-9-inch baking pan with solid vegetable shortening, then dust with flour. Shake out excess flour and set pan aside.
Step 2: To a large mixing bowl, add 1 package (18.25 ounces) plain white cake mix, 1 cup sour cream, 1/2 cup vegetable oil, 3 large eggs and 1 teaspoon pure vanilla extract. With an electric mixer on low speed, blend for 1 minute. Stop the machine and scrape down the sides of the bowl with a rubber spatula. Increase the mixer speed to medium and beat 2 minutes more, scraping the sides down again as needed. The batter should look well-combined and thickened.
Step 3: Pour the batter into the prepared pan, smoothing it out with a rubber spatula. Place pan in oven. Bake the cake until it is light brown and springs back when lightly pressed with your finger, about 35 to 40 minutes.
Step 4: Remove pan from oven and place on wire rack to cool for 20 minutes. Run a dinner knife around the edge of the cake and invert it onto the rack, then invert it again onto another rack so that it is right side up. Allow cake to cool completely, 30 minutes. Frost as desired.