In a large bowl, whisk together the flour, cocoa powder, baking soda and salt.
In a separate large bowl, cream the butter, brown sugar and granulated sugar, with an electric mixer at medium speed, until light and fluffy.
Add the egg, egg yolk and vanilla in 3 separate additions and beat until incorporated. Beat in the flour mixture just until combined.
Fold in the chocolate chips and pistachios.
Form the dough into 18 2-tablespoon size portions, about 2 ounces (57g) each. Form the portions into 2-inch heart shapes. (They don’t have to be perfect, because the cookies are going to spread as they bake.) Place the heart shapes on a tray. Cover and refrigerate for 20-30 minutes.
Meanwhile, preheat the oven to 350ºF (180ºC). Place 6 heart shapes on each cookie sheet, about 3 inches apart. Top each one with a piece of chocolate bar. I placed my chocolate pieces on the upper left of each cookie. Bake the cookies until the tops look set, about 10-11 minutes. Be careful not to over bake the cookies or they’ll be more difficult to shape into hearts.
Remove the cookies from the oven and immediately use a large, metal heart-shaped cookie cutter to shape the hot cookies. Place the cookie cutter onto the cookie sheet over each cookie and slide it up and down a couple of times, then slide it side to side a couple of times. Work quickly before the cookies set. Let the cookies cool on the pan for 2-3 minutes, then transfer them to a cooling rack. Top the cookies with chopped pistachios and rose petals before the melted chocolate sets.