Make the cake

1. Set your oven to 350°F. Rub a 9×13-inch baking pan with butter, covering the bottom, the sides, and the corners so the cake releases cleanly.
2. Grab a medium bowl and whisk the self-rising flour, baking powder, and chopped walnuts until the nuts are evenly coated. Put the bowl to one side.
3. In a large mixing bowl, beat the softened butter with the sugar until the mixture turns pale, light, and fluffy. Stop now and then to scrape down the sides of the bowl so nothing stays unmixed. A stand mixer is my tool of choice, but a hand mixer works fine.
4. Crack in the eggs one at a time, beating well after each. Scrape the bowl between additions so the batter stays smooth.
5. Pour in the oil, then add the espresso powder and the vanilla. Mix until everything looks even.
6. Turn the mixer to low speed. Add the flour mixture a little at a time and mix only until you can no longer see dry flour. Then scrape the sides and the bottom of the bowl with a silicone spatula and give the batter one last fold.
7. Scoop the batter into the buttered pan and spread it evenly into every corner, from one side to the other.
8. Slide the pan into the oven and bake for 20 to 25 minutes, until the center is just cooked. My cake was ready at 20 minutes. Take care not to overbake. This is a thin cake, and it finishes quickly.

Make the coffee fudge icing

1. While the cake bakes, measure out all the icing ingredients and line them up beside the stove. The icing comes together fast, so you won’t have time to measure once you start.
2. As soon as the cake comes out of the oven, put the butter, brown sugar, cream, and espresso powder into a medium saucepan. Warm the pan over medium heat, stirring until the butter melts and the sugar dissolves. Bring the mixture to a full boil and let it bubble for one minute.
3. Lift the pan off the heat. Add the sifted powdered sugar and beat the mixture until it turns completely smooth and glossy.
4. Right away, pour the icing over the hot cake. Start at one corner and sweep back and forth so you cover as much of the surface as possible while you pour. The icing thickens quickly, so a knife may not spread it easily. Use an offset spatula to nudge it into any bare patches, but don’t fuss with it.
5. Leave the cake alone until the icing has set and cooled, then cut it into squares and serve.