In a medium-size bowl, mix flour and baking powder; set aside; in a large bowl, beat butter, sugar and lemon zest until well blended; beat in egg then egg whites; beat in vanilla; gradually add flour mixture beating until well blended; stir in pistachios; divied dough in half; on a lightly floured board, shape each half into a long loaf about 4 cm in diameter; place loaves about 8 cm apart on a greased baking sheet; flatten each loaf to a thickness of about 1 cm; bake in a 180ºC F oven until firm to the touch, 15 to 18 minutes; remove baking sheet from oven; cut hot loaves crosswise into about 1 cm thick slices; turn slices cut side down and spread slightly on TWO baking sheets; return to oven and continue to bake until cookies look dry and are lightly browned, about 10 more minutes; transfer cookies to wire racks and let cool completely; prepare Lemon Icing; spread icing over about 3 cm of one end of each cookie; let stand until icing is firm, about 15 minutes; store airtight. Makes about 4- half dozen cookies. Lemon Icing In a small bowl, stir together the sugar and lemon zest; add just enough lemon juice to give icing a good spreading consistency. ----