First, start by separating all of your egg yolks from the egg whites. Use room temperature eggs and butter for this recipe.
Next, cream the butter and sugar in a stand mixer or in a bowl using a hand mixer. This may take about 3 minutes. You want to achieve a very creamy consistency.
Add in your egg yolks one at a time and the milk little by little while continuing to mix.
Add in the vanilla extract and continue mixing.
In a separate bowl, add flour, baking powder, cinnamon and salt. Whisk together to break up any lumps from the flour.
Preheat the oven to 350 degrees F.
Next, add the flour mixture to the butter, sugar and egg yolk mixture about a 1/4 cup at a time so you don’t end up in a cloud of flour dust. Continue mixing until well incorporated. Make sure to scrape the sides of the bowl throughout the process. Transfer your cake batter mixture to another bowl and clean the mixing bowl you were using.
Add the egg whites to the clean mixing bowl and mix until stiff peaks form.
Fold the egg whites gently into the cake batter.
Grease a 9×13 baking dish with butter or cooking spray.
Next, pour the horchata cake batter into the greased baking dish and bake in a preheated 350 degrees F oven for 20-25 minutes or until a toothpick inserted in the center comes out clean. It may seem like there isn’t enough batter but trust me it will expand beautifully, especially after you add in the horchata tres leches glaze. Once it’s finished baking, let it cool for about 20 minutes before moving on to the next step.
Use a fork or a large toothpick to poke holes into the cake. Try not to go all the way to the bottom so that the glaze sits in the cake and doesn’t leak out.
Add the horchata glaze and homemade whipped cream (see instructions below) and refrigerate for 4 hours or overnight.