• Preheat oven to 350°F. Grease and flour two or three 9-inch cake pans. Set aside.
• In a medium bowl, whisk the flour, baking powder, and salt together; set aside.
  3 cups all-purpose flour, 2 teaspoons baking powder, ½ teaspoon kosher salt
  
• In a large mixing bowl, using a hand mixer, cream the sugar and butter together until light and fluffy, about 2 minutes.
  2 cups granulated sugar, 1 cup unsalted butter
  
• Add the eggs one at a time, mixing well between each addition. Mix in the vanilla for 30 seconds.
  4 large eggs, 1 teaspoon pure vanilla extract
  
• In alternating batches, add the flour and buttermilk to the creamed mixture, beginning and ending with flour mixture. Beat at medium-low speed until blended after each addition, about 30 seconds.
  1 cup buttermilk
  
• Divide the batter between the prepared cake pans (about 713 grams per pan for 2 pans or 475 grams per pan for 3 pans). Bake for 30-35 minutes, until a toothpick inserted in the center comes out clean. Cool on wire racks for 10 minutes, then remove the cakes from the pans and allow them to cool completely on the racks.
  

For the Caramel Frosting

• In a large saucepan set over medium heat, combine the brown sugar, butter, milk, and salt. Allow everything to dissolve together and come to a low boil for 4-5 minutes, stirring constantly.
  3 cups brown sugar, 1½ cups unsalted butter, ¾ cup milk, ½ teaspoon kosher salt
  
• Remove the saucepan from the heat and stir in vanilla. Pour the mixture in a large, heat-proof mixing bowl and allow it to cool for 5-10 minutes until it is warm to the touch.
  1 tablespoon pure vanilla extract
  
• Using a hand mixer, gradually beat in the powdered sugar until the frosting thickens, caramelizes, and is a spreadable consistency.
  3-4 cups powdered sugar
  
• Immediately, spread the nearly cooled frosting between and onto the stacked cake layers. If the frosting begins to be difficult to work with, blend in a little heavy cream until it is spreadable again.