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1
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.
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2
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.
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3
Beat on low speed for 30 seconds, then on medium speed for 2 minutes, scraping down the bowl with a spatula occasionally.
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4
Pour the batter onto the prepared baking sheet and spread it out evenly with a spatula.
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5
Bake until a toothpick inserted into the center of the cake comes out clean, 12 to 15 minutes.
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6
Let cool to room temperature.
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7
Cut out an equal number of heart shapes with a cookie cutter and set aside while you make the filling.
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8
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.
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9
Add the chocolate, milk and vanilla, and beat to incorporate.
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10
Reduce the speed to low and add the powdered sugar and salt.
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11
Once the sugar is incorporated, increase the speed to medium and beat until the filling is fluffy, about another minute.
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12
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.
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13
Top with another heart and gently press down to sandwich.
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14
Repeat with the remaining hearts.
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15
Dust lightly with powdered sugar, and serve.
