Preheat the oven to 350°F (175°C). Grease three 9-inch round cake pans and line the bottoms with parchment paper.

Beat the softened butter until light and fluffy. Add 2 cups sugar and beat for about 5 minutes more.

Add the eggs one at a time, mixing well after each addition.

Add the self-rising flour and 1 cup buttermilk alternately, beginning and ending with the flour. Mix just until combined, then beat in the vanilla.

Divide the batter evenly among the prepared pans. Bake for 25–30 minutes, until the centers are set and a toothpick comes out clean.

Cool briefly in the pans, then turn the layers onto wire racks and cool completely.

For the caramel frosting, combine 2 cups sugar, 1 cup buttermilk, shortening, 1/2 cup butter, and baking soda in a 3- to 4-quart heavy pot.

Cook over medium-low heat, swirling or stirring carefully to keep the mixture moving, until it reaches the soft-ball stage, 235–245°F (113–118°C).

Remove from the heat and beat with a wooden spoon until creamy and spreadable. Work while the frosting is still warm.

Spread the caramel frosting between the cooled cake layers and over the top and sides. Let the frosting set before slicing.