In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl if using a hand mixer, cream the butter, sugar, and brown sugar together until light and fluffy, beating at high speed for about 4 minutes. Add the eggs, one at a time, making sure the first egg is well incorporated before adding the second one.
In a second mixing bowl, whisk the flours, cocoa powder, baking powder, and salt together. Add the dry ingredients to the wet ingredient, mixing at low speed just to incorporate. Using a spatula, fold in the dark chocolate chunks and milk chocolate chips. The mixture will be quite stiff at this stage.
Set the chopped pistachios in a shallow bowl. Portion out the dough by using a large scoop (3 tbsp/45 ml capacity) to make large (3.5-in/8-cm) cookies, or a medium scoop (1 1/2 tbsp/22 ml capacity) to make smaller (2-in/5-cm) cookies. Press the top of each cookie mount in the pistachios, then set on the prepared baking sheets, spacing the cookies 2 in (5 cm) from one another. Use a fork to slightly press down each mound of dough.
Bake each sheet of triple chocolate cookies for 9 to 13 minutes, depending on the size of the cookies, until puffed and just set. Be careful not to overbake the cookies--they should be set around the edge, but still seem quite soft in the center when they come out of the oven. Let the cookies cool on a wire rack, then enjoy.
SERVING: Always bring the baked triple chocolate cookies back to room temperature before serving, or even better, reheat them in a 300°F (150°C) oven for 5 minutes to get that delightful freshly baked flavor again.
STORAGE: Store the triple chocolate cookies in an airtight container at room temperature for up to 4 days. You can also freeze the baked cookies in a zip-top bag or an airtight container for up to 2 months.
MAKE-AHEAD OPTION: Set the mounds of portioned cookie dough close to one another on a single, parchment paper-lined baking sheet. Freeze until hard, then transfer the raw triple chocolate cookie mounds in an airtight container or a zip-top bag and keep frozen until needed. To bake, simply transfer the required amount of cookies on a parchment paper-lined baking sheet and bake for 1-2 minutes more than the indicated time.
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