Swirled Caramel Sheet Cake With Fudgy Chocolate Frosting.

Ingredients

• 2 sticks (1 cup) salted butter, at room temperature
• 1 1/4 cups granulated sugar
• 1 tablespoon vanilla extract
• 3 large eggs, at room temperature
• 2 1/2 cups all purpose flour
• 1 1/2 teaspoons baking powder
• 1/4 teaspoon baking soda
• 1/2 teaspoon kosher salt
• 1 1/3 cups milk, at room temperature
• Caramel Swirl
• 1 1/2 sticks (3/4 cup) salted butter, at room temperature
• 1/2 cup dark brown sugar
• 1/2 cup maple syrup
• sea salt
• 2 cups semi-sweet chocolate chips
• 1 1/3 cups sour cream
• 1 teaspoon espresso powder (optional)