Grease a 9x13-inch pan. Preheat the oven to 350 degrees F.

In a large bowl, whisk together flour, baking powder, cinnamon and salt. Set aside. In a stand-up mixer with paddle attachment, beat egg yolks and sugar until pale yellow and double in volume, about 3 minutes. Mix in the vanilla and the milk on low speed just until combined, then fold the egg yolk mixture into the flour mixture.

Clean and dry the egg yolk bowl. Add the egg whites and beat with the whisk attachment on high until stiff peaks form, about 3 to 4 minutes. Be sure they are stiff peaks or else your cake could deflate.

Fold the egg white mixture into egg yolk and flour mixture.

Pour the batter into the cake pan and bake for 18 to 20 minutes. Let cool completely in the pan before proceeding with the rest of the recipe. The cake may shrink a bit as it cools--that's ok!

After the cake has cooled, using a fork, poke a bunch of holes in the top. Pour the horchata on top; it may seem like a lot of liquid, but the cake will absorb it! Cover lightly with plastic wrap and transfer to the fridge to absorb, about 3 hours, ideally overnight.

When you're ready to serve the cake, add the heavy cream and sifted powdered sugar into the bowl of a stand-up mixer with the whisk attachment (you can also do this with a medium bowl and an electric hand mixer). Beat until soft peaks form, about 3 minutes. Add it to the top of the cake and smear it around. Garnish with a sprinkling of cinnamon (I used a sieve to get proper distribution).