Spread the 1 cup of pecans in a small baking pan and toast in the oven, stirring frequently, for 6-9 minutes, or until lightly browned, be careful not to burn. Immediately turn out into a small bowl. Let stand until completely cooled; set aside.
In a medium bowl, thoroughly stir together the flour, baking soda, and salt; set aside.
In a food processor, process the pecans and brown sugar continuously until the pecans are completely pureed and form a paste, 3-4 minutes -- scraping down sides of processor and bottom several times.
Add the butter and process until well blended. Add the powdered sugar, maple syrup, egg yolk, and vanilla and process until very well blended and smooth. Add the flour mixture and process in on/off pulses just until evenly incorporated.
Divide the dough into thirds. Place each portion between large sheets of wax paper. Roll out each portion to a generous 1/8" thick; check the underside of dough and smooth out any wrinkles that form. Stack the rolled portions, paper and all on a baking sheet.
Freeze for 30 minutes or until chilled and firm.The dough may be frozen for up to 24 hours, let thaw before using.Prehat oven to 350*.Grease several baking sheets or coat with nonstick spray.
Working with one portion at a time and leaving remaining portions chilled, gently peel away, then pat one sheet of wax paper back into place. Flip dough over, then peel off and discard the second piece of paper. Using a.
1 1/2-1 3/4" round or oval, scalloped or fluted cookie cutter, cut out cookies.if at any point the dough softens too much to handle easily, transfer dough to freezer to firm up.Using a spatula, carefully transfer the dough to the baking sheets, spacing about 1 1/2" apart. Reroll any dough scraps. Continue cutting out the cookies until all the dough is used. Lightly sprinkle half the cookies with the finely chopped pecans, patting them down to embed them slightly -- these rounds will be the sandwich tops.
Bake the cookies one sheet at a time in the upper third of the oven for 6-8 minutes or until just tinged with brown and slightly darker around the edges. Reverse sheets halfway through from front to back to ensure even browning.Transfer sheet to wire rack and let set 2-3 minutes so cookies firm up slightly.Using a spatula, transfer cookies to wire rack to cool completely.
in a small heavy saucepan, bring the maply syrup to a simmer over medium-high heat. Simmer briskly for 2 minutes.
Remove from heat.let cool slightly.
Pour the syrup into the food processor. Let stand until barefly warm.
Add the powdered sugar, butter and vanilla and process just until well blended and smooth. Spread a generous measuring teaspoon of buttercream over the underside of each untopped round.Cover each with a pecan-topped round.
Gently press down and then adjust the top if necessary.
Store in an airtight container for up to 1 week or freeze up to one month.