In a large mixing bowl, mix together cake mix, whole milk, butter, vanilla extract, and eggs. Pour cake batter into the prepared pan and bake cake according to package instructions.
Once cake is baked and allowed to cool for 10-15 minutes, use a toothpick or fork to poke holes all over the cake. A little bit at a time, pour milk mixture over the cake, allowing it to soak in before pouring additional mix. Continue until all milk is poured and soaked in.
Use other half of the heavy cream to make whipped cream; mix powdered sugar in slowly as you beat heavy cream in another large bowl with an electric mixer on high speed. Beat until soft peaks form on beaters.