Preheat oven to 350F (175C) and spray a 9x13 glass baking dish with baking spray. Set aside.
Place egg whites in a large bowl or in the bowl of a stand mixer and use an electric mixer or your stand mixer’s whisk attachment to beat until foamy. Gradually increase mixer speed to medium-high and add just 1 cup (200g) of sugar, adding about 2 Tablespoons at a time and allowing about 10 seconds after each addition. Increase mixer speed to high and beat until egg white mixture is thick and glossy and you achieve stiff peaks¹ (much like you are making a meringue).
If you used a stand mixer, scoop egg whites into a separate clean bowl and set aside and carry on using the same bowl for the next step (no need to clean the bowl). If you used an electric hand mixer, just grab a new bowl at this point.
Place softened butter and remaining 1 cup (200g) of sugar in bowl. Beat until creamy and well-combined.
Add egg yolks, one at a time, stirring after each addition.
Stir in vanilla extract.
In a separate bowl, whisk together flour, baking powder, and salt.
Using a spatula and stirring by hand, alternate adding milk and flour mixture to your egg yolk mixture (I usually do this in about 4 parts, about ¼ cup of the milk at a time and ¼ of the flour mixture at a time).
Once milk and flour have been stirred into egg yolk mixture, grab your egg white meringue from earlier and use a spatula to gently fold meringue into batter. Make sure it is completely combined and no white spots of egg remain, but use a gentle hand to not deflate the egg whites or over-mix the batter or you'll have a dense cake.
Spread batter evenly into prepared 9x13 pan and transfer to 350F (175C) oven and bake for 35-40 minutes.
Once cake has finished baking, allow to cool for about 30-60 minutes.
Once cake has cooled (do not invert it, leave it in the cake pan), prepare topping by whisking together condensed milk, evaporated milk, heavy cream, and vanilla extract.
Use a large fork or a wooden skewer to poke holes all over the cake (poke down to the bottom of the cake pan). Pour milk mixture slowly and evenly over cake. Cover and transfer cake to refrigerator and allow to chill for at least several hours or overnight.
Once ready to serve, prepare whipped cream:
Combine heavy cream, powdered sugar, and vanilla extract in a large bowl and use an electric mixer to beat until thickened and you achieve stiff peaks and a Cool Whip consistency.
Spread whipped cream over tres leches cake and sprinkle with ground cinnamon, if desired.
Slice and serve! Enjoy!
¹Stiff peaks simply means that if you lift the beater straight out of the batter, the peak that forms will not recede back or melt back into itself. It should hold a tall, stiff peak, and if you turn the beater upside down the peak that has formed on the end of it should hold its shape as well. See photo in post or video for visual!
How to Store
Store Tres Leches cake covered in the refrigerator. Tres Leches cake will keep for up to 3 days in the refrigerator.