How to Make It
In a large bowl, beat 6 tablespoons of the softened butter with the sugar until light and fluffy. Add the egg yolk, lemon zest and the vanilla and beat until combined. In a medium bowl, whisk the 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 and refrigerate until firm, about 20 minutes.
Preheat the oven to 350°. On a lightly floured surface, roll out the dough a scant 1/4 inch thick. Cut the dough into 4 pieces and transfer to 2 baking sheets. Bake in the upper and lower thirds of the oven for 15 minutes, or until crisp; shift the pans from top to bottom and front to back for even baking. Let cool on a wire rack.
Transfer the pastry to a sturdy resealable plastic bag and crush gently using a rolling pin. Transfer the crumbs to a food processor, add the remaining 6 tablespoons of softened butter and process until moistened. Press the crumbs into a 9 1/2-inch fluted 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.