1.Spray 8-inch square baking pan with cooking spray, then line with parchment paper. Add a tray of water in the oven, about 2 cups, preheat oven to 320°F.
2.In the bowl of a standing mixer with whisk attachment add egg whites and turn to low speed until large bubbles are formed. Add in cream of tartar, whisk until small bubbles are formed. Add in 1/4 cup plus 2 teaspoons sugar gradually, whisk until stiff peaks.
3.In a separate bowl, add egg yolks, salt and 1/4 plus 2 teaspoons sugar, beat with a whisk until thick. Add in oil, beat until combined.
4.In a small bowl add warm water to instant coffee powder, stir until dissolved, add in milk, mix well. Add coffee milk to egg yolk mixture, mix well.
5.Sift together cake flour, baking powder and baking soda, then sift again into coffee egg yolk mixture, mix until combined.
6.Add meringue to batter in 3 batches, mix gently using a balloon whisk. Fold gently with spatula until well mixed, then tap mixing bowl on countertop for a few times to burst large bubbles. Pour batter into baking pan, tap baking pan again for a few times.
7.Bake at 320°F for 25 minutes, then increase to 355°F and bake for another 15 minutes, or until fully cooked, wooden stick inserted in the center of the cake comes out clean. The heat for different ovens is different, the suggested time is only for reference, adjust the baking time based on your oven, if necessary. Do not open the oven door during the first 30 minutes of baking or the cake might collapse when out of oven.
8.Drop baking pan from 1 foot high onto countertop (to minimize shrinkage). Unmold the cake and set aside on wire rack until completely cooled. Cut coffee sponge cake into pieces when fully cooled.