Preheat oven to 350*. Grease several baking sheets or coat with nonstick spray. In a medium bowl, thoroughly stir together the flour, baking soda and salt; set aside. In a food processor, process the butter and sugars in on/off pulses, then continuously until well blended and smooth. Add the egg yolks,lemon zest, lemon juice, vanilla and lemon extract and process until evenly incorporated. Add the flour mixture and process just until thoroughly incorporated; do not over process. The dough should be faily soft to facilitate piping; if it seems to stiff, add up to 4 tbls water, pulsing to incorporate. Spoon the dough into a pastry bag fitted with a 1/2" open star tip. Pipe 1 1/2" wide swirls onto a baking sheet, spacing about 2" apart. Bake the cookies, one sheet at a time, in the upper third of oven for 7-10 minutes, or until just tinged with brown at the edges. Reverse sheets halfway through baking from front to back to ensure even browning. Transfer the sheet to a wirer ack and let stand until the cookies firm up slightly -- 1-2 minutes. Using a spatula, transfer the cookies to wire racks. Let stand until completely cooled.