Line a large baking sheet with parchment paper (if you have multiple sheets, line two, if not be prepared to use this twice).
In the bowl of a stand mixer fitted with the paddle attachment, beat the BROWNED AND COOLED butter, brown sugar, and granulated sugar on medium-low speed until aerated and lighter in color, about 4 to 5 minutes.
With the mixer running on medium-low speed, add the eggs one at a time, mixing until fully incorporated and scraping down the sides and bottom of the bowl after each addition. Add the vanilla and mix to combine.
In a large bowl, whisk together the all-purpose flour, malted milk powder, Diamond Crystal Kosher Salt, baking soda, spices and walnuts. Add these dry ingredients to the mixer bowl and mix on the lowest speed so the mixture doesn't spill everywhere. Continue mixing until the dry ingredients are fully incorporated.
Remove the frozen filling from the freezer.
Working quickly, scoop a HEAPING ⅓-cup measuring cup full of dough (about 2 ounces) and place a portion of the frozen filling in the center. Encase the filling, letting it peak through the top of the dough. Place each completed scoop on the prepared parchment, Continue with all of the dough. Pop these complete dough balls back into the freezer for at least 1 hour and up to 2 days.
Preheat the oven to 350°F.
Place dough balls on the a sheet pan lined with parchment paper, at least 3 inches apart. I bake 5 cookies at a time, so this recipe takes me 3 batches. Bake for 17 to 20 minutes, until the cookies are golden brown and crisp on the outside while still gooey in the center. Remove from oven and let cool.
