Line a 15 x 10 jelly roll pan with parchment paper. Preheat oven to 370 F.
Sift together flour and baking powder in a bowl and set aside. Whisk the egg whites using high speed of stand mixer. Once frothy, add in the cream of tartar. Once soft peaks form, gradually add the ¼ cup of sugar until the egg whites form shiny, stiff peaks. A good indicator: turn the bowl upside down and the egg whites should not spill or flow.
Meanwhile, beat together the egg yolks and sugar using a wire whisk. Once it is thick and light colored, add in the milk, vegetable oil and the coffee . Add in the flour mixture and whisk together until the mixture is smooth.