Chocolate Mayonnaise Cake

INGREDIENTS

• 1 1/3 cups mayonnaise (do not use reduced-fat or fat-free)
• 1 cup (packed) dark brown sugar
• 2 3/4 cups all purpose flour
• 1 1/2 cups (3 sticks) unsalted butter, room temperature
• 2 ounces bittersweet chocolate (do not exceed 61% cacao), chopped
• 1 3/4 cups boiling water
• 1/4 teaspoon baking powder
• 1 1/4 teaspoons baking soda
• 1 cup sugar
• 3 cups powdered sugar
• 2 large eggs
• 1 teaspoon vanilla extract
• 10 ounces bittersweet chocolate (do not exceed 61% cacao), chopped
• 2/3 cup unsweetened cocoa powder
• 1 tablespoon vanilla extract