Grease an 8x6x2-inch baking dish* with nonstick spray.
Beat the butter and sugar together in a medium bowl with an electric mixer until light and fluffy. Beat in eggs in 2 additions.
Add flour and espresso powder and mix well.
Stir in ¼ cup walnuts by hand.
Scrape batter into prepared baking dish, spreading evenly.
Place in microwave and cook at full power for 2 minutes.
Reduce power to 50% and microwave again for 2 minutes.
After the 4 minutes, check to see if cake is cooked – it should have risen, will spring back when touched and a toothpick should test clean when inserted in the center of the cake.
If it needs a bit longer, microwave at 50% power for 1 minute more at a time, checking for doneness after each minute.
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