Whisk together flour, baking powder, and salt in a small bowl .
Beat together butter and sugar in a large bowl with an electric mixer at
medium-high speed until pale and fluffy. Beat in egg and vanilla. Reduce
speed to low, then add flour mixture and mix until just combined. Stir in
pistachios and lemon zest.
Form dough into a 12-inch log (2-inches in diameter) on a sheet of plastic wrap and
roll up dough in plastic wrap. Chill dough on a baking sheet until firm, at
least 4 hours.
Put oven racks in upper and lower thirds of oven and preheat oven
to 375 degrees F.
Cut enough 1/8-1/4 inch thick slices from log with a heavy knife to fill 2
ungreased large baking sheets, arranging slices about 1-inch apart (chill
remainder of log, wrapped in plastic wrap). If garnishing with coarse sugar,
sprinkle slices with it.
Bake cookies, switching position of sheets halfway through baking, until
edges are golden, 12-15 minutes total. Cool on sheets 3 minutes, then
transfer with a metal spatula to racks to cool completely. Make more
cookies with remaining dough on cooled baking sheets.
Mix Bittersweet Chocolate Sauce, butter, and confectioner's sugar until
smooth. Drizzle over the cooled cookies.