Beat the butter in a large bowl with an electric mixer until smooth. Add the icing sugar and beat until smooth and fluffy. Beat in the egg yolks one at a time, add the salt, lemon zest and the poppy seeds. Beat just until incorporated, then add the flour and beat again until well incorporated. Turn the dough out onto a piece of wax paper and form into a ball. Divide the dough in half and wrap each piece in plastic wrap and refrigerate for 30 minutes.
Form each piece into a log that is 1¼ inch thick, wrap each log in plastic wrap and chill for 2 hours.