Preheat the oven to 350 F and prepare a 9x13 baking pan with cooking spray.
In a medium sized mixing bowl, whisk together the cake four, granulated sugar, cocoa powder, baking soda, baking powder, and salt.
In a large mixing bowl, whisk together the milk, vegetable oil, eggs, and vanilla until combined.
Add the flour mixture into the milk mixture and gently stir with a rubber spatula until combined, being careful not to overmix the batter.
Pour into the prepared 9x13 pan.
Bake in the preheated oven for 30-35 minutes, or until the center springs back when lightly pressed and a toothpick inserted into the center of the cake comes out mostly clean. (While the cake is baking, make the milk filling).
As soon as you pull the cake from the oven, use a toothpick to poke holes all over the cake.
To make the chocolate milk filling:
In a medium mixing bowl, whisk together the ingredients for the milk filling.
Pour evenly over the cake as soon as you have removed the cake from the oven and poked holes in it.
Allow to cool completely.
Once cool, place the pan in the fridge to sit for at least 3 hours before frosting.
To make the chocolate whipped topping:
When the cake is completely cool, make the whipped topping.
In a medium sized mixing bowl, whisk together the powdered sugar and cocoa powder.
Add the heavy cream and use a handheld electric mixer to beat the mixture until fluffy and spreadable with soft peaks.
Notes
Store cake covered in the refrigerator for up to 3 or 4 days. For best results, store without whipped cream topping. Cake freezes well.Frost before serving.