Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper to prevent sticking and to ensure even baking.
Toast Pecans
Toast the pecans in a dry skillet over medium heat until fragrant, about 2 to 3 minutes - be careful not to burn. Let them cool completely before chopping them finely using a food processor.
Cream Butter & Sugar
In a stand mixer or with a hand mixer, cream the butter and powdered sugar at medium-high speed until light and fluffy, about 2 to 3 minutes. Make sure the butter is soft before starting for easier creaming.
Scrape down the sides of the bowl occasionally to ensure even mixing.
Add Flavorings
Add the milk, salt, and vanilla extract to the butter mixture. Beat on medium speed until fully incorporated and smooth (a few small lumps are okay).
Incorporate Dry Ingredients
Reduce the mixer speed to low and gradually add the chopped nuts and flour. Mix just until the dough comes together. Do not overmix, as this can make the cookies tough.
Form Cookies
Scoop tablespoon-sized portions of dough (a cookie scoop helps a lot here) and roll them into smooth balls using your hands. Note that the batter is only mildly sweet but the cookies will be sweeter after rolling in the powdered sugar after baking.
Place the dough balls on the prepared baking sheet, leaving at least 1 inch of space between each ball to allow for slight spreading.
Bake
Place the baking sheets in the center of the oven and bake for 15 to 20 minutes, rotating the sheets halfway through to ensure even baking.
The cookies are done when they are lightly golden brown around the edges and firm to the touch. Do not overbake, as they will continue to firm up as they cool.
Roll in Sugar Twice
While still warm, roll each cookie generously in additional powdered sugar. Let the cookies cool and then roll them again in powdered sugar.
Storage & Freezing
These cookies can be made ahead of time. Store them in an airtight container at room temperature for up to a week.
To freeze, flash freeze on a baking tray, then place in an airtight container and store in the freezer for up to 2 months. Thaw at room temperature and roll in more powdered sugar before serving.
Notes
Note 1: If you don't have a food processor, you can use any nut flour in place of finely chopped nuts.