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 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. Beat until egg whites are thick and glossy.
In the same bowl, beat softened butter and remaining sugar until creamy. Add egg yolks one at a time, stirring after each addition, followed by vanilla extract.
In a separate bowl, whisk flour, baking powder, and salt. Alternate adding milk and flour mixture to your egg yolk mixture. Gently fold meringue into batter.
Spread batter into prepared pan and bake for 35-40 minutes until done.
Prepare topping by whisking together condensed milk, evaporated milk, heavy cream, and vanilla extract. Poke holes in the cooled cake and pour milk mixture over it. Chill for several hours or overnight.
Make whipped cream, spread over cake, and sprinkle with cinnamon if desired. Slice and serve!