Preheat the oven to 350°F. Line baking sheets with parchment paper and set them aside.

In a large bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set the mixture aside.

In the bowl of a stand mixer with the paddle attachment, cream the butter with the granulated sugar and brown sugar for 3 minutes or until light and fluffy. Add the eggs one at a time, mixing the last one fully before adding the next egg. Mix in the vanilla extract.

In three batches, add the flour mixture, making sure it mixes in all the way before adding the next batch. Scrape down the sides as needed. Stir in the chocolate chips.

Portion the cookie dough into 12 even-sized portions, about 5 ounces each. Roll them into a smooth ball and place them on the prepared baking sheets about 6 inches apart. They will spread quite a bit, so you will need to bake them in batches. Gently press down on the cookie dough balls and flatten them out into a large flat circle.

Bake for 12-14 minutes until they appear dry in the center of the cookies.

Let the cookies cool completely on the sheet trays, then transfer them to a wire rack.

While the cookies are cooling, make the frosting. Place the butter into the bowl of a stand mixer with the paddle attachment or with a hand mixer. Beat on medium speed until smooth.

Add the melted chocolate and mix until combined. Then, add the powdered sugar a little at a time until fully combined, scraping down the sides as needed. Add the vanilla, mixing to combine. Add 1 tablespoon of the heavy cream, mixing until fully combined, then whip it on medium-high speed until light and fluffy, about 3 minutes. If it is too thick to pipe, add the additional tablespoon of heavy cream, or more until you reach the desired consistency. 

Using an open star tip, pipe rosettes onto the tops of the cooled cookies.

Serve immediately.