• ¾ cup (1½ sticks) unsalted butter, room temperature, cut into pieces, plus more for pans
• 3 cups (345 g) cake flour
• ¾ cup (150 g) granulated sugar
• 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt
• 4 large eggs, room temperature
• 2 large egg yolks, room temperature
• 1½ cups buttermilk, room temperature
• 1 Tbsp. vanilla extract
• 4 cups (440 g) powdered sugar
• 1½ cups (126 g) unsweetened Dutch-process cocoa powder
• 1½ cups (3 sticks) unsalted butter, room temperature
• ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt
• ¼ cup plus 2 Tbsp. (or more) buttermilk, room temperature