TOAST PECANS: Preheat the oven to 350 degrees F. Spread the pecans in an even layer on a sheet pan. Bake for 5-7 minutes or until pecans smell fragrant. Watch carefully; you don't want to burn them. Remove and allow to cool completely before transferring them to a mini food processor or cutting board. Pulse on the "chop" setting or finely chop. You want very small pieces. Avoid over-pulsing in the food processor or you'll end up with pecan butter.
WET INGREDIENTS: Either in a bowl of a stand mixer with the whisk attachment or a large bowl (and using a hand mixer), combine butter and 3/4 cup powdered sugar. Cream together until smooth and creamy, about 3-4 minutes. Add in the vanilla, peppermint, salt, cornstarch, pulsed/chopped pecans, and 1/4 cup crushed peppermint candies. Beat until smooth and combined. Add in the flour and beat slowly until combined and smooth. It will seem sandy and crumbly at first, but keep mixing; it will come together. Once combined, do not overmix.
CHILL: Wrap the dough in plastic wrap and put it in the fridge for 1 to 2 hours to chill (at least 1 hour).
BAKE: After chilling, measure cookie dough using a 1 tablespoon measuring spoon (each ball should be exactly 1 tablespoon). If the dough gets warmed from being rolled or sitting in a warm kitchen, put in the freezer for 10-15 minutes. Preheat the oven to 375 degrees F. Line a light-colored sheet pan with a Silpat liner (Note 4) Bake for 7-11 minutes (mine always take 8 minutes). Remove from the oven, let stand on tray for 3-5 minutes and then transfer to a wire cooling rack. Let cool for 2-3 or so minutes.
POWDERED SUGAR: Add powdered sugar to a bowl (I start with 3/4 cup; you may need slightly more). Add remaining 1/4 cup crushed peppermint candy. Stir together; roll the warmed cookie in the powdered sugar and then return to the wire cooling rack. Let cool down a few more minutes and then roll in the powdered sugar/peppermint mixture again.
STORAGE: Store in an airtight container at room temperature. Re-roll in additional powdered sugar as needed. Best enjoyed within 1-3 days.