Velvety Chocolate Mayonnaise Cake with Fudge Frosting

Method

1. Grease a 9x13-inch baking pan with butter and set it aside.
2. In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt until well combined. Set aside.
3. In a glass measuring cup, dissolve the instant coffee in the hot water. Mix well and set aside.
4. In a large bowl, use an electric mixer to beat the mayonnaise with the white and brown sugars until the mixture is pale and creamy.
5. Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl with a spatula as needed. Stir in the vanilla extract.
6. Add one-third of the dry ingredients to the mayonnaise mixture, followed by half of the hot coffee mixture. Beat until just combined. Repeat with the second third of the dry ingredients and the remaining coffee. Finally, add the last third of the dry ingredients and mix until the batter is smooth but not overmixed.
7. Pour the batter into the prepared pan and bake at 350°F for 28 to 30 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
8. While the cake cools, prepare the frosting. In a microwave-safe bowl, melt the chopped chocolate in 20-second intervals, stirring between each, until smooth. Let it cool slightly.
9. In a large bowl, beat the unsalted butter with an electric mixer until creamy and fluffy. Gradually add the powdered sugar, starting on low speed and increasing to high, beating for 2-3 minutes until light and fluffy.
10. Add the cocoa powder to the butter mixture and beat until combined, scraping down the sides as needed.
11. Incorporate the cooled melted chocolate and heavy cream into the frosting. Beat until smooth and well combined. Chill the frosting for 10 minutes before using.
12. Spread the chilled frosting evenly over the completely cooled cake. Use a spatula or the back of a spoon to create decorative swirls.