1 3/4 cups/225 grams all-purpose flour
1/2 cup/45 grams unsweetened Dutch-processed cocoa powder
1 1/2 teaspoons baking powder
1 1/2 teaspoons baking soda
1/2 teaspoon fine sea salt
1 1/2 cups/300 grams granulated sugar
2 eggs, at room temperature
1 cup/240 milliliters whole milk
1/2 cup/120 milliliters grapeseed oil or any mild-flavored oil
1/2 teaspoon pure vanilla extract
1 cup/240 milliliters boiling water
FOR THE TRES LECHES:
1 (12-ounce) can/355 milliliters evaporated milk
1 (14-ounce) can/395 milliliters sweetened condensed milk (3 tablespoons reserved for the whipped cream)
1 cup/240 milliliters heavy cream
Scraped seeds of 1 vanilla bean or 1 teaspoon vanilla extract (optional)
FOR THE CONDENSED MILK WHIPPED CREAM:
1 cup/240 milliliters heavy cream, chilled
1/2 teaspoon vanilla extract
1 teaspoon cinnamon (optional)
Chocolate shavings (optional)
Heat the oven to 350 degrees. Grease the bottom only of a 9-by-13-inch cake pan (preferably metal), leaving a 1/4-inch border that remains ungreased. Do not butter the sides of the pan.
Place a large sifter or a sieve in a large bowl. Add the flour, cocoa, baking powder, baking soda and salt and sift. Add the sugar and whisk to combine.
In another large bowl, whisk the eggs, milk, oil and vanilla until combined. Gradually add the wet ingredients to the dry ingredients and whisk until there are no lumps and the batter is smooth. Carefully pour in the boiling water and stir until combined.
Pour the batter into the prepared pan. Bake in the center of the oven until a wooden skewer inserted in the center comes out clean and the cake bounces back when lightly pressed, about 50 minutes.
Meanwhile, make the tres leches: In a large bowl, combine the evaporated milk, condensed milk (setting 3 tablespoons aside for the whipped cream), heavy cream and vanilla bean seeds or extract (if using) and whisk to combine. Cover with plastic wrap and refrigerate if not using immediately.
Remove the cake from the oven and let stand for 10 minutes. Run a butter knife around the cake’s edges. Cool completely.
While the cake is still in the pan, use a skewer or a fork to poke small holes through the cake to the bottom, about an inch apart. Slowly spoon the tres leches over the top of the cake, giving it time to soak in. Once the liquid is absorbed, repeat until all the tres leches is used. (Toward the end, the tres leches will pool at the edges, but don’t worry; it will be absorbed while in the fridge.) Cover the cake with plastic wrap and refrigerate for at least 5 to 6 hours, preferably overnight.
When ready to serve, make the whipped cream: Add the reserved condensed milk along with the cream, vanilla and cinnamon (if using) to the bowl of an electric mixer. Beat on medium high until the cream has doubled in volume, forms soft peaks and is light and fluffy. Spread the cake with the cream and sprinkle the chocolate shavings on top. Serve from the pan.