Toast the pecans in a dry skillet for just a few minutes, keeping them moving and DON'T WALK AWAY or they will burn.
Allow the toasted pecans to cool completely and then add them to a food processor. Pulse until they become finely ground.
Add the sugar, flour and salt and pulse again until combined.
Add the butter, flax "egg" and vanilla. Pulse until the dough looks like a crumbly, wet sand texture. Wrap in plastic and chill for at least 30 minutes.
Preheat oven to 325 degrees.
Lightly flour your work surface and roll the dough to about 1/4" thickness.
Cut the dough with a 2" round cookie cutter and place them on a lined baking sheet about an inch apart.
Place a pecan halve on the top of each cookie, pressing lightly. Finally, dust with powdered sugar before baking.
Bake for about 20 minutes, but give them a quick check at about 16-18 minutes. Every oven bakes differently.
Allow the cookies to cool completely on a wire rack and store in an airtight container.