In a medium mixing bowl, whisk together the flour, dark cocoa powder, cornstarch, baking soda, and salt; set aside.

In a large mixing bowl, use an electric mixer to beat together the melted butter, granulated sugar, brown sugar, and vanilla until well combined.

Add the eggs and beat on low speed, just until incorporated.

Pour the dry mixture into the wet and mix on low speed until combined.

Line baking sheets with parchment paper. Roll the dough into 1-inch balls and place on the baking sheets, leaving 2 inches of space between each dough ball.

Place the cookie sheets of dough balls in the refrigerator to chill for 30 minutes.

While the dough is chilling, preheat the oven to 350 F.

Once the dough balls are finished chilling, bake at 350 F for 9-10 minutes until set around the edges. Remove from the oven and cool on the cookie sheets for 5 minutes then transfer to wire cooling racks.

Pour the semi-sweet chocolate chips and heavy cream into a microwave safe bowl and heat on high for 30 seconds. Stir well, then heat for another 20-30 seconds until the chocolate is melted. Continue to stir until fully combined and smooth.

Spoon the chocolate frosting onto the center of each cookie and spread out towards the edges.