Start by melting the semisweet and unsweetened chocolate together with the butter using a double boiler or microwave. Set aside to cool slightly.
In a large mixing bowl, beat the eggs, sugar, instant espresso powder, and vanilla extract with an electric mixer until light and fluffy.
Slowly add the melted chocolate mixture to the wet ingredients and mix until well combined.
In a separate bowl, whisk together flour, baking powder, and kosher salt. Gradually fold into the wet ingredients until just combined.
Gently fold in the semisweet chocolate chips, ensuring even distribution throughout the dough.
Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes.
Preheat the oven to 350°F (175°C) and line baking sheets with parchment paper.
Scoop tablespoon-sized portions of dough onto prepared baking sheets, spacing them apart. Bake for 10-12 minutes until edges are set but centers remain soft.
Remove cookies from oven and cool on baking sheets for 5 minutes before transferring to a cooling rack. Once cool, sandwich two cookies together with desired filling.