At casual Southern gatherings--book clubs and prayer meetings and such--ladies come toting sheet cakes. They're all nestled in their pans, usually Pyrex, and ready to have fat squares cut right out of them. They're the warmest welcome. Neither fussy nor fancy, but always yummy. This yellow vanilla cake tastes good enough to serve plain with tea, but you can pour caramel all over it too.
Preheat the oven to 350 degrees F. Butter and flour a 9-by-13-inch cake pan.
Combine the buttermilk and vanilla in a medium bowl. Combine the flour, the sugar, baking powder, and salt in the bowl of a stand mixer. Beat with the paddle attachment on low speed until well mixed. Add the oil and beat until evenly distributed. With the machine running, add the butter 1 tablespoon at a time and beat until fully incorporated. The mixture will look like coarse sand.
Add the eggs one at a time, beating well after each addition and scraping the bowl occasionally. While beating, add the buttermilk mixture in a slow, steady stream. Beat just until smooth. Pour the batter into the prepared pan and spread evenly.
Bake until a tester inserted in the center comes out clean, about 25 minutes.
Let cool completely in the pan on a wire rack. Pour as much caramel as desired over the cake.
Set the butter in a large saucepot over medium heat. When it's halfway melted, add the sugar and salt. Cook, stirring with a wooden spoon, until the mixture is dark golden brown. It will look grainy and the fat will separate from the sugar and then come back together.
Remove from the heat and add the evaporated milk while whisking. Be careful! The caramel will bubble up. Scrape up any sugar that's stuck to the bottom of the pan. Make sure the sugar is completely dissolved.
Set over low heat and cook, whisking often, until very smooth and creamy, about 1 hour. Remove from the heat and cool completely, stirring occasionally.
The cake keeps at room temperature covered tightly for up to 3 days.