Powder threads and work into butter. Cream butter and sugar, add egg yolks and zest and blend mixture well. Add salt to flour and sift into batter to form dough. Wrap dough and chill for 1 hour. On a floured surface, roll dough out and cut with sharp edged glass or cookie cutter into two inch rounds. Place on greased cookie sheet and bake at 190ºC for about 10 minutes until very lightly browned. Cool on a rack.

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