Place the bittersweet chocolate and butter in a large heatproof bowl set over a pot of simmering water and melt, stirring frequently, until smooth. Remove the pan from the heat.
In a stand mixer fitted with the paddle attachment (or using a handheld electric mixer), beat the eggs and sugar on medium speed until thick and pale, approximately 5 minutes. Add the vanilla and salt. Use a rubber spatula to fold in the melted chocolate.
Sift together the flour, espresso powder and baking powder. Fold into the batter, followed by the chocolate chips. Pour the batter into a shallow baking dish and place in the freezer until firm, approximately 50-60 minutes.
Preheat the oven to 350 degrees F and line 2 large baking sheets with parchment paper. Fill a large cup with hot water. Dip a 2-tablespoon cookie scoop into the hot water, shake off the excess liquid and then scoop the dough onto the prepared sheet. Leave about 2 inches of space between each cookie so they have room to spread. Repeat, dunking the scoop in the hot water as needed.
Bake the cookies for 10-12 minutes until dry around the edges and cracked on top. Allow to cool completely before serving.