Preheat the oven to 350°F. and grease a 9 x13-inch baking pan with cooking or baking spray.
In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, salt and cinnamon together and set aside.
Next, separate the egg yolks from the egg whites and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl if using a handheld mixer, beat the egg yolks with 3/4 cups of sugar on medium-high speed until smooth and creamy, about 2 minutes.
Add the milk and the vanilla extract and beat until combined, scraping down the sides and up the bottom of the bowl with a rubber spatula as needed.
With the mixer on low speed, slowly add the dry ingredients mixing until just combined.
In a separate bowl, using a handheld mixer or a stand mixer with the whip attachment, whip the egg whites on medium-low speed until foamy. Switch to medium-high and gradually add the remaining 1/4 of sugar. Whip until stiff peaks form.
In 2 or 3 additions, gently fold the whipped egg whites into the cake batter with a rubber spatula. Don’t overmix to avoid deflating the fluffy whipped egg whites.
Pour the batter into the prepared pan and bake for 25-30 minutes or until a toothpick inserted in the middle of the cake comes out clean.
Remove from the oven and cool the cake completely.