Preheat oven to 325°F. Beat butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add sugar, beating at medium speed until light and fluffy. Add eggs, 1 at a time, beating just until blended after each addition.

Add flour to butter mixture alternately with buttermilk, beginning and ending with flour. Beat at low speed just until blended after each addition. Stir in vanilla. Pour batter into a greased (with shortening) and floured 10-inch (16-cup) tube pan.

Bake at 325°F for 1 hour and 5 minutes to 1 hour and 10 minutes or until a long wooden pick inserted in center comes out clean. Cool in pan on a wire rack 10 to 15 minutes; remove from pan to wire rack, and cool completely (about 1 hour). Serve with Buttermilk Custard Sauce.

Make the Buttermilk Custard Sauce: Whisk together 2 cups buttermilk, 1/2 cup sugar, 1 Tbsp. cornstarch, and 3 large egg yolks in a heavy 3-qt. saucepan. Bring to a boil over medium heat, whisking constantly, and boil 1 minute. Remove from heat, and stir in 1 tsp. vanilla extract. Serve warm or cold. Store leftovers in an airtight container in refrigerator for up to 1 week.