I always start with the caramel because it needs time to cool and thicken before it goes anywhere near the cake. Warm a heavy-bottomed saucepan over medium heat, then add the sugar a quarter cup at a time, letting each addition melt before adding more. Stir gently until it turns a deep amber color, then whisk in the butter, followed by the cream and salt. Pour it into a container and let it cool completely while you move on.
Mix your dry ingredients together, then cream in the butter until the mixture looks like damp sand. Add the pumpkin puree and egg whites, followed by the buttermilk, oil, and vanilla. Divide the batter between your prepared pans and bake until a toothpick comes out with just a few moist crumbs. Let the layers cool in the pans before turning them out.
While the layers cool, beat the butter and cream cheese together until smooth and light. Mix in the vanilla and salt, then slowly add the powdered sugar and spices. Finally, fold in the cooled caramel until the frosting turns a soft, warm tan color.
Place your first cake layer on a board and spread on a layer of frosting. Pipe a border around the edge to keep the caramel from sliding out, then drizzle caramel in the center. Chill briefly before adding the next layer and repeating.
Once all the layers are stacked, cover the entire cake in a thick layer of frosting. Smooth it out or add texture with the back of a spoon, then finish with a sprinkle of flaky sea salt for that irresistible sweet-and-salty bite.