Pistachios are the star of this cake. Instead of a butter cake with the occasional bite of pistachio, this is a dense, nutty cake completely permeated by nuts. Replacing most of the flour with ground pistachios gives the cake a coarse, almost chewy texture and intense flavor. It's lovely without, but if you'd like you can also top it with a simple lemon glaze made by combining lemon juice and confectioners' sugar.
Preheat the oven to 350 degrees F. Butter the bottom of an 8 1/2- by 4 1/2-inch loaf pan. Line the bottom with parchment paper, then butter and flour the entire pan. Pulse 1 cup of the pistachios and 1/4 cup of the sugar in a food processor until very finely ground. Transfer the pistachio mixture to a large bowl. Whisk in the flour, baking powder, baking soda and salt.
In a large bowl, beat the butter and the remaining 3/4 cup sugar with an electric mixer on medium speed until pale and fluffy, about 2 minutes. Beat in the eggs, one at a time, and beat until combined. Beat in the lemon zest. In three additions, beat in the pistachio-flour mixture, alternating with additions of buttermilk, starting and ending with pistachio-flour mixture. Transfer the batter to the prepared pan and smooth the top. Bake until a toothpick inserted into the center of the cake comes out with moist crumbs attached, 50 to 60 minutes.
Transfer the pan to a rack to cool for 15 minutes. Run a knife around the edge of the pan to loosen the cake and then tip the cake out of the pan. Remove the parchment, turn the cake right side up, and let it cool completely. Sprinkle the remaining pistachios on top of the cake before serving.