Place 8 ounces of chocolate chips and the butter in a microwave safe bowl.
Microwave the chocolate and butter mixture in 30 second increments until the chocolate is almost melted. Stir until smooth.
Place the eggs, brown sugar and vanilla extract in the bowl of a mixer. Beat for 3-4 minutes or until everything is light and fluffy.
Add the melted chocolate to the egg mixture, beat for 1 minute or until smooth.
Add the flour, salt and baking powder to the bowl. Mix until just combined.
Stir in approximately ¾ of the remaining chocolate chips, reserving the rest for later use.
Cover the bowl and chill the dough for at least 1 hour, or until firm.
Preheat the oven to 350 degrees F. Line two baking sheets with nonstick baking mats or parchment paper.
Form the dough into 2-tablespoon sized balls. Place each ball 2-3 inches apart on the prepared baking sheets.
Press the reserved chocolate chips into the top of each cookie dough ball.
Bake one sheet of cookies at a time for 12-14 minutes, or until edges are set.
Cool the cookies on the sheet for 10 minutes. then transfer to a rack to cool completely. Serve immediately, or store in an airtight container for 5 days.