For the whoopie pies: Position an oven rack in the center of the oven and preheat to 350 degrees F. Line a 12-by-18-inch rimmed baking sheet with parchment paper and spray lightly with nonstick cooking spray.
In the bowl of an electric mixer fitted with the paddle attachment, combine the cake mix, eggs, 1 cup water, vegetable oil, vanilla and salt. Beat on low speed for 30 seconds, then on medium speed for 2 minutes, scraping down the bowl with a spatula occasionally. Pour the batter onto the prepared baking sheet and spread it out evenly with a spatula.
Bake until a toothpick inserted into the center of the cake comes out clean, 12 to 15 minutes. Let cool to room temperature. Cut out an equal number of heart shapes with a cookie cutter and set aside while you make the filling.
For the filling: In the bowl of an electric mixer fitted with the paddle attachment, beat the cream cheese on medium speed until soft, about 1 minute. Add the chocolate, milk and vanilla, and beat to incorporate. Reduce the speed to low and add the powdered sugar and salt. Once the sugar is incorporated, increase the speed to medium and beat until the filling is fluffy, about another minute.
To assemble the whoopie pies, use a small spatula or spoon to gently spread a generous tablespoon of the filling over the surface of one of the hearts. Top with another heart and gently press down to sandwich. Repeat with the remaining hearts. Dust lightly with powdered sugar, and serve.
This recipe has been updated and may differ from what was originally published or broadcast.