Preheat the oven to 350 degrees. Grease a 9 x 13 inch baking pan with 1 tablespoon of oil. Sprinkle 3 tablespoons of flour and dust the surface of the pan with the flour, tap out the excess flour and discard.

Mix the remaining 2 cups of flour and the baking powder in a bowl. Set aside.

Combine the oil, coffee, and maple extract in another bowl. Set aside.

Beat the egg yolks and brown sugar with an electric mixer on high speed until pale in color and fluffy, about 3 to 4 minutes. Reduce the speed to low and add ⅓ of the flour mixture. Mix until just incorporated. Add ½ of the coffee and oil mixture. Mix until just incorporated. Add another ⅓ of the flour mixture and mix until just combined. Add the remaining ½ of the oil and coffee mixture, mixing until just combined. Add the remaining flour mixture and mix until combined into a cake batter consistency. 

Using a clean dry bowl and whisk attachment to an electric mixer, beat the egg whites on medium high speed until soft peaks form, about 3 to 5 minutes. Add the granulated sugar 1 tablespoon at a time, mixing after each addition. Once all the sugar is mixed into the egg whites, increase the speed to high and beat until stiff peaks form, about 2 to 3 more minutes.

Fold ¼ of the sugar and egg whites mixture into the cake batter. Add the remaining egg white mixture into the cake batter in 3 more additions folding after each addition. Fold the chopped walnuts into the batter.

Gently pour the batter into the prepared baking pan and smooth the top evenly. Transfer into the oven and bake for about 35 to 40 minutes or until the cake is golden and a cake tester comes out clean.

Allow the cake to cool in the baking pan and dust with powdered sugar (if using). Slice into 3 inch squares and serve.