1) Prep your pan and oven
Heat oven to 350°F. Lightly grease a 9x13-inch baking dish. (No need to flour it, we want the soak to slide down the sides.)
Pan note: Glass can bake a little slower than metal. Start checking at the early end of the bake time, then give it a few extra minutes if needed.
2) Mix dry ingredients
In a medium bowl, whisk together the flour, baking powder, and salt.
3) Whip the egg whites
Make sure your bowl and beaters are clean and grease-free. In a large bowl with a hand mixer (or stand mixer), beat the egg whites on medium-high until they look foamy. Gradually add 3/4 cup of the granulated sugar and beat until you get glossy, stiff peaks. Set aside.
4) Beat yolks, then combine
In another bowl, beat the egg yolks with the remaining 1/4 cup sugar until pale and thick, about 2 minutes. Mix in the whole milk and vanilla.
Fold the yolk mixture into the whipped whites gently, using a spatula. Then sprinkle the dry ingredients over the top and fold just until you do not see dry streaks. Do not overmix. A few tiny lumps are better than a deflated batter.
5) Bake
Pour batter into the prepared dish and smooth the top. Bake 28 to 33 minutes, until the top is lightly golden and the center springs back when gently pressed.
Doneness check: A toothpick inserted near the center should come out clean or with a few tender crumbs.
6) Cool, then poke
Cool the cake in the pan for 20 to 30 minutes. You want it warm, not hot, so it can drink up the milk without turning fragile.
Then use a fork or skewer to poke holes all over the surface, going deep and evenly across the whole cake.
7) Mix and pour the three milks
Whisk together evaporated milk, sweetened condensed milk, whole milk, and optional vanilla and cinnamon. Slowly pour the mixture over the cake, a little at a time, letting it absorb between pours. (Yes, it looks like a lot. Trust the process.)
A little pooling around the edges is normal. Most of it absorbs as the cake chills.
8) Chill until set
Cover and refrigerate for at least 6 hours, ideally overnight, so the soak fully settles into the cake.
9) Whip the topping
Right before serving, beat cold heavy cream, powdered sugar, and vanilla to soft to medium peaks. Soft peaks droop gently. Medium peaks hold their shape but the tip curls. Spread over the chilled cake. Finish with a light dusting of cinnamon if you like.
10) Slice and serve
Slice into squares and serve cold. Add berries if you want a bright, fresh contrast to all that creamy sweetness.
