Prep the pan: Heat the oven to 350°F (or as directed on your cake mix).
Grease a 9x13-inch baking pan.
Mix the cake: Prepare the cake batter according to the box, but use milk instead of water and add 1 teaspoon vanilla for more flavor.
Bake: Pour the batter into the pan and bake as directed until a toothpick comes out clean. Do not overbake.
Cool slightly: Let the cake cool in the pan for about 10–15 minutes. It should be warm, not hot.
Poke holes: Using a fork or skewer, poke holes all over the cake, spacing about 1/2 inch apart.
Go deep, but don’t tear the cake.
Make the tres leches: In a bowl, whisk 1 cup whole milk, 1 can sweetened condensed milk, and 1 can evaporated milk until smooth.
Soak the cake: Slowly pour the milk mixture over the entire cake, focusing on the edges and dry spots. The cake will look very wet—this is right.
Chill: Cover and refrigerate for at least 4 hours, preferably overnight, so the cake absorbs the milk fully.
Whip the topping: Beat 1 1/2 cups heavy cream with 2–4 tablespoons sugar and 1 teaspoon vanilla to medium-soft peaks.
Frost and finish: Spread whipped cream over the chilled cake. Dust with a little cinnamon and add berries if you like.
Slice and serve cold.