Preheat oven to 350°F (175°C). Lightly grease a 9 × 13-inch (23×33 cm) baking pan.
Sift the flour, cocoa powder, baking powder, baking soda, salt, and cinnamon into a medium bowl. Whisk to distribute the cocoa, cinnamon, and leaveners evenly.
1 cup (120 g) all-purpose flour, ¼ cup (25 g) unsweetened cocoa powder, 1 tsp (4 g) baking powder, ½ tsp (2.5 g) baking soda, ½ tsp (3 g) fine sea salt, 1 tsp (2.5 g) ground cinnamon
In a large bowl, whisk the sugar, eggs, and oil until smooth and slightly thickened. Whisk in the milk and vanilla extract until incorporated.
1 cup (200 g) granulated sugar, 3 large eggs, ⅓ cup (80 ml) canola oil, ½ cup (120 ml) whole milk, 1 tsp (5 ml) vanilla extract
Add the dry ingredients to the wet ingredients and stir only until no streaks of flour remain. Avoid overmixing, which can make it harder for the baked cake to absorb the tres leches mixture evenly.
Pour the batter into the prepared pan and spread it evenly. Bake for 25 to 30 minutes, or until the center springs back when lightly pressed and a toothpick inserted in the center comes out with a few moist pieces of cake attached.
While the cake is still warm, use a skewer or fork to poke holes evenly over the entire cake, going all the way to the bottom of the pan. Deep holes give the tres leches mixture a path through the full height of the cake.
Whisk the cocoa powder into the sweetened condensed milk until completely smooth. Whisk in the evaporated milk and whole milk until evenly mixed. Adding the cocoa to the condensed milk first makes it easier to incorporate without leaving dry pockets of cocoa in the thinner milk mixture.
2 tbsps (10 g) unsweetened cocoa powder, 1 (14 oz /396 g) can sweetened condensed milk, 1 (12 oz /354 ml) can evaporated milk, ¾ cup (180 ml) whole milk
Slowly pour the chocolate tres leches mixture over the warm cake, working across the entire surface. Pause as needed to let the milk absorb before continuing rather than pouring it all on at once.
Cool the cake completely, cover, and refrigerate for at least 4 hours or overnight for the best results. The longer refrigeration time allows the three-milk mixture to distribute throughout the cake before the topping is added.
When ready to finish the cake, beat the cold heavy cream with an electric mixer until soft peaks form. Sift in the confectioners’ sugar and cocoa powder, add the vanilla extract, and continue beating until stiff peaks form. Stop once the whipped cream holds its shape.
2 cups (480 ml) cold heavy cream, ⅓ cup (40 g) confectioners' sugar, ¼ cup (25 g) unsweetened cocoa powder, 1 tsp (5 ml) vanilla extract
Spread the chocolate whipped cream evenly over the chilled cake. Dust with cocoa powder before serving, if desired.
unsweetened cocoa powder