Line 2 large cookie sheets with parchment paper.

In a large bowl, whisk together the flour, cocoa powder, baking soda and salt until well blended.

In a separate large bowl, cream the butter, brown sugar and granulated sugar, with an electric mixer at medium speed, until pale and fluffy.

Beat in the egg, egg yolk and vanilla in 3 separate additions, making sure each one is incorporated before adding the next. On low speed, mix in the flour mixture just until combined. Mix in the chocolate chips and pistachios.

Form the dough into 18 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 dough portions on each cookie sheet, about 3 inches apart. Top each one with a piece of chocolate. 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, then 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.