Combine the butter and sugars in the bowl of a stand mixer fitted with a paddle attachment. Beat on medium speed until light and fluffy. Beat in the eggs and vanilla.
In a separate bowl, whisk together the flour, salt and baking powder. On low speed, add the flour mixture to the butter mixture in a few additions, scraping the bowl and beater in between. Do not over mix. Stir in the walnuts and the three types of chopped chocolate. Cover the bowl and refrigerate for at least 1 hour to firm.
Preheat the oven to 350 degrees F. Line three baking sheets with parchment paper or use non-stick baking sheets.
Using an ice-cream scoop or a slightly heaping tablespoon, drop mounds of the cooled cookie dough about 2 inches apart on the prepared baking sheets.
Bake for 12 to 15 minutes until the bottoms are brown, and the chocolate pieces have melted. Underbake for chewy cookies, or bake to the higher end of the time range for crispier cookies. Remove to a rack to cool completely. Store in an airtight container for up to 3 days.
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