3 1/2 cups all-purpose flour
1 teaspoon salt
1 1/2 cups unsweetened cocoa powder
1 tablespoon baking soda
1 teaspoon baking powder
1 cup (2 sticks) unsalted butter, room temperature
2 cups sugar
2 large eggs
2 cups buttermilk, room temperature
2 teaspoons pure vanilla extract
Preheat oven to 400 degrees. Sift together flour, salt, cocoa powder, baking soda, and baking powder. Set aside. Line 2 baking pans with Silpat parchment paper. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar. Add eggs, buttermilk, and vanilla. Beat until well combined. Add the dry ingredients in three batches. Mix until combined.
Pour whoopie-pie batter into two separate gallon-size Ziploc plastic bags. Snip a 1/2-inch hole in one corner of bag. Pipe Vs onto parchment-lined baking sheets, spaced 2 inches apart. Bake for 12-15 minutes and cool.
3/4 cup plus 1 tablespoon sugar
1/3 cup water
1 tablespoon light corn syrup
3 large egg whites
1/4 teaspoon pure vanilla extract
Red food coloring to make the frosting the pink or red of your choice
Heat 3/4 cup sugar, the water, and corn syrup in a medium sauce pan over medium heat, stirring until sugar dissolves. Increase heat, and bring to a boil. Cook until it registers 235 degrees on a candy thermometer.
Meanwhile, whisk whites with a mixer on medium speed until soft peaks form, about 2 1/2 minutes. Add remaining tablespoon sugar; reduce speed to medium-low. Pour sugar syrup into whites in a slow, steady stream. Increase speed to medium high; beat until stiff peaks form, about 7 minutes. Beat in vanilla at the end. Spread on half your hearts and sandwich with the other have of the hearts.