Step 1 Heat the oven to 350 degrees. In a saucepan over low heat, combine the unsweetened chocolate, bittersweet chocolate and butter, stirring occasionally until melted. Remove from the heat.
Step 2 Combine the eggs, espresso and sugar in the bowl of an electric mixer and mix on low speed, scraping the bowl as needed, until blended, then on high speed until the mixture becomes lighter in color and thickens, 2 to 3 minutes. Add the melted chocolate mixture (do not cool it beforehand) and beat on medium speed until just mixed with the batter.
Step 3Add the pastry flour, baking powder and salt and beat at medium-low until just mixed in, scraping down the sides of the bowl. Remove the bowl from the mixer and fold in the chocolate chunks.
Step 4Spoon one-fourth cup batter per cookie onto parchment-lined baking sheets. Bake until set, about 15 minutes. Cool for a few minutes, then remove to a rack to cool completely.