Preheat oven and prepare cake mix according to package directions for a 9×13 sheet cake.

When you pull the cake out of the oven, use the handle of a wooden spoon to poke holes throughout the cake while it is still hot. Immediately pour the evaporated milk, sweetened condensed milk, and half and half over the cake, taking care to drizzle some in each hole.

Let the cake cool on the counter for about an hour, then chill for another hour while you whip the cream with the sugar and vanilla. Once the cake is cool, spread whipped cream over the top and sprinkle cocoa with a sifter to garnish.

Chill for at least two hours before serving. This cake actually tastes better the longer you let it cool, so if possible try to budget a few extra hours of cooling time before serving. Store in the fridge for up to five days.