The Nostalgic Marshmallow-Filled Chocolate Cake

Ingredients

• 2 cups all-purpose flour
• 1 cup Dutch-process cocoa powder
• 1 1/2 cups granulated sugar
• 1/2 cup packed dark brown sugar
• 1 3/4 teaspoons baking powder
• 1 teaspoon baking soda
• 1 teaspoon kosher salt
• 3 large eggs
• 1 cup buttermilk
• 1 cup freshly brewed coffee
• 1/4 cup vegetable oil
• 2 teaspoons vanilla extract
• 2 large egg whites
• 1/4 teaspoon cream of tartar
• 3/4 cup granulated sugar
• 1/2 cup light corn syrup
• 1/4 cup water
• 1/2 teaspoon kosher salt
• 1/2 cup unsalted butter
• 1 tablespoon vanilla extract or vanilla paste
• 2 cups powdered sugar
• 1/4 cup whole milk
• 2 teaspoons light corn syrup
• 1 teaspoon vanilla extract
• 3 ounces dark bittersweet chocolate (about 60 percent cacao)
• 1 cup powdered sugar
• 2 teaspoons whole milk