Preheat oven to 350 degrees F (175 degrees C). Grease a 13x9 inch baking dish with cooking spray.
Using a stand mixer, beat the egg whites with the whisk attachment until they are stiff and form peaks. This may take a minute or two. Pour the egg whites into another bowl and place them in the refrigerator until you're ready to add them to the batter.
Using the stand mixer bowl again, cream together the butter, sugar and vanilla extract.
In a separate bowl, whisk together the flour, baking powder and salt.
Add the flour mixture to the butter/sugar mixture alternately with the milk.
Add the stiffly beaten eggs to the cake batter. Fold the egg whites in gently. Do not overmix at this point. If you do, your cake will become more dense.
Pour batter into baking dish. Bake 30-40 minutes or until toothpick comes out clean in centre. Cool completely before frosting.
Make the frosting:
Beat the butter, peanut butter and vanilla together until smooth.
Add in the powdered sugar and mix again.
Add the milk a tablespoon at a time until the frosting reaches the correct consistency.
Spread on cooled cake. Sprinkle with chocolate chips.
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**