Blackout Chocolate Cake

Name: Blackout Chocolate Cake
Cuisine: American
Ingredients: Unsalted butter, room temperature (for pans); ¾ cup Dutch-processed cocoa powder, plus more for pans; 1½ cups all-purpose flour; 1½ teaspoons baking soda; ¾ teaspoon baking powder; 1 large egg, room temperature; 1 large egg yolk, room temperature; 1½ cups (packed) dark brown sugar; ¾ cup sour cream, room temperature; 5 tablespoons vegetable oil; 2 teaspoons vanilla extract; ¾ teaspoon kosher salt; ⅓ cup Dutch-processed cocoa powder; 5 tablespoons cornstarch; 1¼ cups heavy cream; 4 large egg yolks, beaten to blend; 1¼ cups whole milk; ⅔ cup (packed) dark brown sugar; ¼ teaspoon kosher salt; 4 ounces milk chocolate, chopped; 1 teaspoon vanilla extract; 1 cup heavy cream; 3 tablespoons golden syrup (such as Lyle’s Golden Syrup); ⅛ teaspoon kosher salt; 12 ounces bittersweet chocolate, chopped; 2 tablespoons unsalted butter, room temperature