Preheat the oven to 350°F and line two baking sheets with parchment paper. Bake the heart cookies according to package instructions, then let them cool completely.
In a medium bowl, beat the softened butter until smooth. Mix in 1 cup of powdered sugar, then add the remaining powdered sugar, heavy cream, vanilla, and cocoa powder. Beat until light and fluffy.
Transfer the frosting to a piping bag with an open tip (or simply spread it with a knife). Lay the cookies out, separating the pink ones from the plain ones. Pipe or spread a generous dollop of frosting onto the back of a plain cookie, then gently press a pink cookie on top.