Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan. In a large mixing bowl, combine the all-purpose flour, cocoa powder, baking powder, baking soda, salt, and ground cinnamon. Whisk together until well mixed and set aside.

In the bowl of a stand mixer, beat the egg yolks and granulated sugar on medium speed until creamy and pale, about 5 minutes. Add the whole milk and vanilla extract, mixing well. Gradually incorporate the dry ingredients.

In a separate clean bowl, whip the egg whites using an electric mixer on medium-high speed until foamy. Gradually add the remaining sugar and continue to whip until stiff peaks form, about 3-5 minutes. Gently fold the whipped egg whites into the chocolate batter in three additions.

Pour the batter into the prepared baking pan and bake for 25-30 minutes or until a toothpick comes out clean. Let cool in the pan for 10-15 minutes, then remove and cool completely.

While the cake cools, whisk together the evaporated milk, sweetened condensed milk, whole milk, and additional cocoa powder in a bowl until fully combined. Set aside.

Once the cake has cooled, poke holes all over the surface and gradually pour the soaking mixture over the cake. Cover with plastic wrap and refrigerate for at least 2 hours, ideally overnight.

Whip the heavy cream with sugar and cocoa powder until stiff peaks form. Spread the whipped cream over the soaked cake and garnish with shaved chocolate, fresh mint, or ground cinnamon.