Preheat oven to 350°F (175°C). Grease a 9x13 inch baking dish generously with butter or nonstick spray, then lightly dust with flour, tapping out any excess.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
Separate the eggs, placing yolks in a large bowl and whites in a separate clean, dry bowl. Beat yolks with 3/4 cup sugar on medium-high for 2-3 minutes until pale yellow and thickened. Add whole milk and vanilla, mix on low just until combined.
Add the flour mixture to the egg yolk mixture in two batches, gently folding with a rubber spatula after each addition. Stop as soon as no dry streaks remain; do not overmix.
Using a clean, completely dry bowl and whisk attachment, beat egg whites on medium speed until foamy. Gradually add remaining 1/4 cup sugar while beating until stiff, glossy peaks form.
Add about a third of the whipped egg whites into the batter, folding gently to lighten. Fold in remaining egg whites in two more additions, just until no white streaks remain. Batter should be light, airy, and pale yellow.
Pour batter into prepared baking dish and smooth the top. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean and the top springs back lightly when touched. Cool in pan for 15 minutes.
Whisk together evaporated milk, sweetened condensed milk, and 1/4 cup heavy cream in a large measuring cup or bowl until fully combined and smooth.
While cake is still just warm, poke holes all over the surface with a wooden skewer or fork, spacing about 1 inch apart. Slowly pour the milk mixture over the cake in sections, allowing liquid to soak in evenly. Cover and refrigerate at least 3 hours, preferably overnight.
Once cake is fully chilled, beat remaining 1 cup heavy cream with powdered sugar until soft, billowy peaks form. Be careful not to overbeat.
Spread whipped cream evenly over the top of the chilled cake using an offset spatula. Dust lightly with ground cinnamon. Slice into squares and serve chilled.