To prepare the cake, cube the butter and place it in a medium, light-colored, heavy-bottomed saucepan over medium low heat. Stir regularly to allow the butter to melt evenly. It will begin to sizzle, foam, and then brown. Continue stirring and scraping the sides of the pan as the butter browns and remove to a heat-safe bowl when the butter has golden flecks throughout and the pan smells fragrant. Allow to cool completely to soft, room temperature. I prefer to speed this process up in the fridge.
When ready to bake the cake, ensure that your butter is room temperature and preheat the oven to 350 degrees. Lightly grease two 9” pans with baking spray and cut out rounds of parchment paper to fit directly into the bottom of each pan. Cream the butter, sugar, and brown sugar in a large bowl or the bowl of a stand mixer on medium speed for 4 minutes. The mixture will be light and fluffy. Scrape the sides of the bowl and add the eggs one at a time, stirring after each addition. Add half of the flour, the powder, soda, and salt. Sitr on low to combine. Combine the milk, vinegar, and vanilla and add half of that mixture. Stir on low and scrape the sides of the bowl. Add the remaining dry ingredients followed by the remaining wet and use a rubber spatula to fold in any unincorporated bits. Do not over mix. Divide the batter between the two pans and bake until a toothpick inserted comes out clean, about 35 minutes. Bake time differs from oven to oven. Remove from oven when done and allow to cool completely.
In the meantime, prepare the caramel. In a medium, heavy-bottomed pan, combine the butter and brown sugar and stir regularly until the mixture has melted. Bring to a rolling boil and use a rubber spatula to barely stir the caramel on the edges of the pan toward the middle. Cook for 4 minutes. Remove from heat and carefully stir in the remaining ingredients. Transfer to a heat-safe container and allow to cool completely.
When ready to frost the cake, cream the butter for the buttercream on medium speed for 3 minutes. Add the caramel and powdered sugar and whip for an additional minute. Scrape the sides of the bowl and mix again to incorporate.
Use a serrated knife to level the cakes and place a schmear of buttercream on a prepared cake board or plate. Place the first layer on top and smooth a large plop of buttercream on this first cake. Place a cup of the buttercream in a piping bag or Ziploc and snip the end off to make a ½” sided opening. Pipe a dam around the perimeter of the top of the cake and spoon some of the remaining caramel inside. Place the second cake layer on top and lightly press down to seal it to the bottom cake. Be sure no caramel smushes out. Finish icing with the remaining buttercream and decorate as desired! I used toasted hazelnuts and extra caramel!
Recipe created for Carandini by Kate Wood of @ katie_clova