Preheat the oven to 325 °F (163 °C) and lightly grease a 9×13‑inch pan.
In a bowl, whisk the cake mix, eggs, melted butter, milk and vanilla until the batter is smooth.
Pour the batter into the prepared pan and bake for about 25 minutes, or until a toothpick comes out clean.
Let the cake cool completely, then spread chocolate frosting (microwave 20 seconds for a pourable consistency).