Bumpy Cake

Preparation - Preheat the oven to 350°F. Grease a 9x13-inch cake pan with cooking spray. Line the bottom of the pan with parchment paper to ensure the cake doesn't stick to the pan.

Cake - Make the cake batter: In a mixing bowl or the bowl of a stand mixer, add the granulated sugar, brown sugar, flour, cocoa powder, baking powder, baking soda, and salt. Mix to combine until the mixture is free of any large clumps. Mix in the eggs and oil, then mix in the buttermilk and vanilla extract. Add the brewed coffee and mix until just combined and the batter is smooth and free of any streaks of the dry ingredients.

Baking - Bake: Pour the cake batter into the prepared pan. Bake until a toothpick inserted into the center comes out clean, 22 to 25 minutes. Set it on a cooling rack to cool in the pan for 30 minutes, and then transfer it to the freezer to chill for 20 minutes.

Frosting - While the cake chills, make the frosting: In the bowl of a stand mixer, beat the butter on medium speed until pale and fluffy. Mix in 2 cups of powdered sugar, the milk, vanilla extract, and salt on medium-low speed. Gradually mix in 2 1/2 cups of powdered sugar on low speed until the mixture comes together. Turn the speed up to medium and whip the frosting until very thick but fluffy, about 1 minute. You can add up to 1/2 cup of powdered sugar if the mixture isn't thick enough.

Assembly - Create the bumps: Once the cake is very cold, add the frosting to a piping bag or a zip-top plastic bag. Snip the corner making a hole an inch wide. Use the piping bag to pipe thick lines of frosting down the length of the cake, leaving about an inch between each line. Use any extra frosting to add additional bumps, or freeze the leftovers in an airtight container for future use. Return the cake to the freezer for at least 30 minutes while you make the fudge topping.

Fudge - Make the fudge topping: In a saucepan on medium heat, melt the butter. Mix in the sugar, cocoa powder, buttermilk, corn syrup, and salt. Whisk until smooth and bring the mixture to a rolling boil. Cook for 3 minutes without stirring. Remove from the heat and whisk in the sifted powdered sugar and vanilla extract until smooth. Let the topping cool slightly for 10 minutes so it's not too hot for pouring.

Finishing - Add the fudge topping and chill: Remove the frozen cake from the freezer, stir the fudge, and quickly pour it all over the top. Gently tilt the cake pan to move the fudge around to cover the whole top of the cake. Move quickly since the fudge topping will set up as it chills. You can use an offset spatula to help nudge it around. Return the cake to the freezer to set for another 15 minutes before slicing and serving. Store leftovers in an airtight container in the fridge for up to 5 days.