Preheat oven to 350 degrees F. Grab a 9x13 glass pan and set aside. Do NOT grease the pan.
Dry ingredients: Whisk all-purpose flour, baking powder, ground cinnamon, and salt. Sift the dry ingredients into another bowl. Set aside.
Wet ingredients: In another bowl, combine the egg yolks, 3/4 cup granulated sugar, and vanilla extract. Using a handheld mixer or stand mixer, beat until light and fluffy, about 3-4 minutes. Add 1/2 cup of milk, and mix.
In another bowl, on medium-high speed, beat the egg whites until soft peaks form. Add the remaining ¼ cup granulated sugar and continue beating until you get stiff peaks. Set aside.
Add to the dry ingredients to the wet ingredients, and mix until just combined. Then, fold in the whipped egg whites into the batter until just combined. Do not overmix or the whipped egg whites will deflate.
Pour batter into the UNGREASED 9x13 glass baking pan until evenly distributed. Bake in the middle rack for about 20-25 minutes, until a toothpick or cake tester inserted in the middle comes out clean. Once ready, remove from oven and let it cool for about 30 minutes.