Preheat oven to 350℉. Line a 6 inch baking dish with parchment paper or grease it.
To a medium bowl, add in the eggs and beat them. Its very important to SIFT IN the powdered milk and baking powder to the eggs. Whisk together vigorously to incorporate well for a minute or two. Next add in the orange zest, salt and sugar (if using) and whisk to combine. Pour your batter into the pan and bake for 27-30 minutes or until toothpick comes out clean. Allow to set in pan for 10 minutes then turn it out onto a cooling rack to fully cool.
Notes
Nutritional Info includes only first five ingredients (does not include sugar, vanilla and chocolate topping).