脆饼饼干：Line 2 rimmed baking sheets with parchment. Whisk together flour and salt. In the bowl of a mixer fitted with the paddle attachment, beat butter until fluffy, 3 to 5 minutes. Add sugar, and continue to beat until pale and fluffy, occasionally scraping down the sides of bowl, about 2 minutes more. Beat in vanilla. Add flour mixture, and mix on low speed, scraping sides if necessary, until flour is just incorporated and dough sticks together when squeezed with fingers.