Preheat oven and spray a 13 x 9 x 2-inch baking pan with cooking spray.

In a large bowl, whisk together flour, sugar, cocoa, baking powder, and salt until well combined.

Add nonfat milk, unsweetened applesauce, and vanilla extract to the dry ingredients.

Whisk until thoroughly blended, ensuring no lumps remain.

Pour the cake batter into the prepared baking pan and spread evenly.

In a separate bowl, whisk together boiling water, brown sugar, and cocoa powder until the sugar and cocoa are fully dissolved.

Carefully pour the pudding mixture over the cake batter in the baking pan. Note that the pudding layer will be thin and runny.

Bake in the preheated oven for 35-40 minutes, or until the top of the cake is firm to the touch.

Let the cake rest for 15 minutes before cutting it into slices.

Cover and refrigerate any leftovers for up to 7 days, or wrap tightly and freeze for up to 2 months.