Preheat oven to 350ºF and grease a 9x13 baking dish with butter; set aside.
In a large bowl, whisk together flour, baking powder, salt, cocoa powder, and ground cinnamon; set aside.
In the bowl of a stand mixer, add egg whites, and with the whisk attachment, process on medium-high speed until soft peaks form.
Slowly add ¼ cup granulated sugar, and continue beating for 5 to 6 minutes or until the meringue form stiff peaks; set aside.
In a medium bowl, add yolks and the remaining 1 cup granulated sugar. Beat on medium-high speed until the mixture turns light yellow and gets fluffy, about 1 to 1:30 minutes.
Stir in whole milk and vanilla extract; beat until just combined.
Pour egg yolk mixture into the dry ingredients, and fold until just combined using a spatula.
Add meringue to the mixture and gently fold until just combined – don’t overdo it.
Pour batter into prepared pan and even it out.
Transfer to the preheated oven and bake for 25 to 30 minutes or until a cake tester comes out clean with just a few crumbs. Transfer to a wire rack to cool.