In a large saucepan, melt chocolate chips and butter over low heat, stir until smooth. Remove from heat; cool slightly.
In a small bowl, whisk the eggs, sugar, vanilla and if desired , espresso powder and stir until blended. Whisk into chocolate mixture. In another bowl, mix the flour, baking powder, salt; add to chocolate mixture, mix well. Fold in chocolate chunks.
Drop by 1/4 cupfulls, 3 inch's apart on parchment paper lined cookie sheets. Bake at 350 for 12 to 14 minutes or until set. Cool on sheets for 1 to 2 minutes. Remove to racks to cool.