Make the cake batter: Preheat oven to 350°F (175°C). Grease and flour two 9-inch cake pans. Cream the butter and granulated sugar together until light and fluffy, about 4 minutes. Add eggs one at a time, mixing well after each addition.

Add dry and wet ingredients alternately: Whisk together flour, baking powder, and salt in a separate bowl. Add about a third of the flour mixture to the batter, mix briefly, then add half of the buttermilk. Repeat, ending with the last of the flour mixture. Stir in vanilla. Do not overmix once the flour is added.

Bake: Divide the batter evenly between the two prepared pans. Bake for 25-30 minutes, until a toothpick inserted in the center comes out clean. Let the cakes cool in their pans for 10 minutes, then turn out onto a wire rack to cool completely before icing.

Cook the caramel icing: Melt the butter in a medium saucepan over low heat. Add the brown sugar and whisk to combine. Bring to a gentle boil over medium heat and let it bubble for about 5 minutes, whisking constantly.

Add the buttermilk: Slowly and carefully pour in the buttermilk or evaporated milk while whisking, it will bubble up when it hits the hot sugar mixture. Continue stirring until the mixture returns to a full, rolling boil, then remove from heat and stir in the salt.

Finish the icing: Transfer the hot caramel mixture to a stand mixer bowl. With the mixer on low, gradually add the sifted powdered sugar until smooth and glossy. Mix in the vanilla extract last.

Ice the cake immediately: This icing sets as it cools, so work quickly. Spread about 1 cup of icing between the two cake layers, then cover the top and sides with the remaining icing while it's still warm and pourable. If it starts to harden before you finish, microwave it in 10-second bursts to loosen it back up.