Preheat oven to 350°. In a large saucepan, melt chocolate chips and butter over low heat, stirring until smooth. Remove from the heat; cool until mixture is warm.
In a small bowl, whisk the eggs, sugar, vanilla and, if desired, espresso powder until blended. Whisk into chocolate mixture. In another bowl, mix the flour, baking powder and salt; add to chocolate mixture, mixing well. Fold in chocolate chunks; let stand 10 minutes or until mixture thickens slightly.
Drop by 1/4 cupfuls 3 in. apart onto parchment paper-lined baking sheets. Bake 12-14 minutes or until set. Cool on pans 1-2 minutes. Remove to wire racks to cool. Yield: about 1-1/2 dozen.