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.
FOR THE FROSTING:.
In a food processor, process the powdered sugar and lemon zest until the zest in evenly incorporated, about 2 minutes. Add the butter, and process until very well blended - and smooth. Add 1 1/2 tsps lemon juice and food coloring, if using, and process till smooth and well blended. If necessary, add a bit more powdered sugar to stiffen the frosting or a bit more lemon juice to thin it so that is of piping consistency. Spoon the frosting into a pastry bag fitted with 1/2 open star tip. Pipe 1-1 1/2 frosting swirls onto the cookies. let stand until the frosting completely sets - at least 1 hour. Store in a single layer or layered with wax paper in an airtight container in a cool place for up to 2 days.Refrigerate for up to 1 week, or freeze for up to 2 months.