In a large bowl, beat 6 tbls. of the softened butter with the sugar until light and fluffy. Add the egg yolk, lemon zest and vanilla and beat until combined. In a medium bowl, whisk flour with the pecans, baking powder, cinnamon and salt. Add to the wet ingredients and beat just until combined. Flatten the dough into a disk, wrap in plastic wrap and refrigerate until firm, about 20 minutes.
Preheat the oven to 350*F. On a lightly floured board, roll out the dough about 1/4" thick. Cut the dough into 4 pieces and move to 2 baking sheets. Bake in the upper and lower thirds of the oven for 15 minutes, or till crisp. Shift the pans from top to bottom and front to back for even baking. Let cool on a wire rack.
Move the pastry to a heavy resealable plastic bag and crush gently using a rolling pin(I use a drinking glass). Move the crumbs to a food processor, add the remaining 6 tbls. softened butter and process until moistened. Press the crumbs into a 9" tart pan with a removable bottom, forming an even layer all around. Pack the crumbs tightly. Refrigerate the tart shell until chilled, about 15 minutes. The crust can be kept in an airtight container for 2 days. Fill with desired filling.