Preheat the oven to 350°F. Line baking sheets with parchment paper or silpat mats.
Place the chocolate chips and butter in a large, heatproof (metal or pyrex) bowl set over a pan of simmering water. Stir until melted and smooth.
In another bowl, whisk together the flour, baking powder and salt; set aside.
In a large bowl, use an electric mixer to beat the eggs, brown sugar and vanilla on high speed until light and fluffy. Reduce the speed to low and beat in the melted chocolate mixture. Then mix in the dry ingredients and blend just until combined. Stir in the chocolate chunks, but leave out the optional, additional chunks.
Drop the dough by heaping tablespoonfuls onto the prepared baking sheets, keeping a round shape as much as possible; place 2 to 3 inches apart. If you're using the extra chopped chocolate bar, place 2 to 3 chunks on top of each cookie.
Bake until the cookies are shiny and crackly, yet still soft in the center, 12 to 14 minutes. Cool on the baking sheets for about 10 minutes before transferring the cookies to a cooling rack.
Notes
Nestle sells packages of "chocolate chunks" in the baking aisle of your grocery store.
You can certainly substitute chopped chocolate for chips.
These cookies contain a lot of chocolate, and are therefore extremely chocolatey and rich. You can cut the recipe in half to make just 2 dozen cookies if you don't wish to invest in so much chocolate!