Position the racks in the upper and lower thirds of the oven and preheat the oven to 350°.
Line two cookie sheets with parchment paper.
In a small bowl, mix the flour, baking powder, and salt together thoroughly; set aside.
Place the 8 ounces of chocolate and the butter in the top of a double boiler of barely simmering water and stir frequently just until melted and smooth.
Remove the top pan from the heat. Leave the bottom of the double boiler on the heat.
In a large heatproof bowl, whisk the eggs, sugar, and vanilla together thoroughly.
Set the bowl over the double boiler and stir until the mixture is lukewarm to the touch.
Stir the eggs into the warm (not hot) chocolate.
Stir the flour mixture, then the chocolate chunks into the batter.
Scoop slightly rounded Tablespoons of batter 1 ½" apart onto the cookie sheets.
Bake at 350° until the surface of the cookies look dry and set but the center is still gooey, about 12-14 minutes.
Let set on the pans on wire racks for 2 minutes, then slide the parchment paper onto the racks and let cookies cool completely. Store in a tightly sealed container.