1 | Whisk together flour, baking powder and salt in a small bowl. |
2 | 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. |
3 | Form dough into a 12" log (2" 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. |
4 | Put oven racks in upper and lower thirds of oven and preheat oven to 375F degrees. |
5 | Cut enough 1/8"- to 1/4"-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. |
6 | 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. |
7 | Mix White Chocolate Orange Sauce, butter and confectioners sugar until smooth. Drizzle over the cooled cookies. |