Instructions

1. Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
2. In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt until well combined.
3. In a large bowl, beat the softened butter and granulated sugar together with a hand mixer or stand mixer until the mixture is light and fluffy, about 3-5 minutes.
4. Add the egg and vanilla extract to the butter-sugar mixture, and beat until well combined.
5. Gradually add the dry ingredients to the wet mixture in three parts, alternating with the milk. Start and finish with the dry ingredients, mixing until just combined.
6. Using a cookie scoop or spoon, drop equal portions of the batter onto the prepared baking sheet, spacing them about 2 inches apart.
7. Bake for 10-12 minutes, or until the cakes are set and a toothpick inserted into the center comes out clean.
8. Remove the baking sheet from the oven and let the cakes cool completely on the sheet.
9. In a separate large bowl, beat the heavy cream and powdered sugar together until stiff peaks form, creating a fluffy filling.
10. To assemble the whoopie pies, spread a generous amount of the filling onto the flat side of one chocolate cake, then place another cake on top, flat side down, creating a sandwich.
11. Repeat with the remaining cakes and filling. Serve immediately, or store in an airtight container in the refrigerator for up to 3 days.