Step 1

Heat the oven to 350°F. Lightly grease a 13x9-inch pan with cooking spray.

Step 2

In a medium bowl, whisk the boiling water and cocoa powder to combine. Add the whole milk and whisk to combine. Add the eggs, oil, and vanilla extract and whisk well to combine.

Step 3

In a large bowl, whisk the granulated sugar, flour, baking powder, baking soda, and salt to combine. Gradually add the cocoa mixture, whisking well until the batter comes together and is smooth.

Step 4

Pour the batter into the prepared pan and spread in an even layer. Bake for 35 to 40 minutes, until a tester inserted into the center comes out clean or with a few moist crumbs attached. Let cool.

Step 5

In a medium pot, whisk the dry milk, granulated sugar, cornstarch, cocoa powder, and salt to combine. Add the whole milk, whisking to combine. Cook over medium heat, whisking constantly, for 1 to 2 minutes, until the mixture simmers and thickens. Remove the pot from the heat. Whisk in the butter and ½ teaspoon of the vanilla extract.

Step 6

Using a chopstick or the rounded handle of a wooden spoon, poke holes about three-quarters of the way into the cake in 1-inch increments, until the whole cake has holes throughout.

Step 7

Gently pour the warm pudding into the holes, filling each one. Cover the pan with plastic wrap and refrigerate the cake for 4 hours.

Step 8

In a medium bowl, whip the cream to soft peaks, then add the powdered sugar and the remaining 1 teaspoon of the vanilla extract and whip to medium peaks.

Step 9

Spread the cream evenly over the surface of the cake. Refrigerate until ready to serve.

Step 10

Do Ahead: The cake can be made 2 days ahead. Keep refrigerated.