Preheat the oven to 350°F. Line 2 large cookie sheets with parchment paper and set aside.
In the bowl of a stand mixer or in a large mixing bowl with a hand mixer, beat the partially melted butter and granulated sugar until well combined and slightly fluffy, about 2 minutes.
Add the eggs and cake batter flavoring and beat until the mixture is thick, pale, and smooth.
Mix in the pink food coloring until the batter is a deep pink shade, slightly darker than you want the final cookies to be.
In a separate bowl, whisk together the flour, cornstarch, baking soda, and salt until evenly combined.
With the mixer on low speed, add the dry ingredients to the wet ingredients in 2 additions, alternating with the buttermilk, beginning and ending with the dry ingredients, and mix just until a thick, smooth batter forms.
Transfer the batter to a large piping bag fitted with a round tip, or use a large heavy-duty zip-top bag with the corner snipped off.
Pipe heart shapes onto the prepared baking sheets by starting at the bottom point of the heart, piping up and around the left side, then up and around the right side, letting the two curves meet at the top. Leave about 2 inches of space between each heart.
Bake the hearts, one sheet at a time, for 10 to 12 minutes, until they puff slightly, the tops look set, and the centers feel firm when lightly tapped.
Remove the pans from the oven and let the cookies cool completely on the baking sheets before filling.
While the cookies cool, make the filling. In a clean mixing bowl, beat the softened butter until smooth and creamy.
Add the powdered sugar 1 cup at a time, beating on low speed after each addition. Add a few tablespoons of the heavy whipping cream as needed to keep the mixture smooth.
When all the powdered sugar is incorporated, add the remaining heavy cream and the clear vanilla bean paste. Beat on low until combined, then switch to high speed and whip until the frosting is thick, fluffy, and holds stiff peaks.
Pair the cooled cookies by matching hearts of similar size and shape. Place one cookie from each pair upside down.
Pipe the frosting in a heart shape onto the center of each upside-down cookie, staying slightly away from the edges.
Gently press the matching cookie on top to create whoopie pies, allowing the filling to spread to the edges.
Serve immediately or chill the whoopie pies in an airtight container in the refrigerator for up to 4 days.