Step 1Make the cake: Heat oven to 350 degrees. Butter the bottom and sides of an 8-inch round cake pan (2 inches deep). Line the bottom with parchment paper and butter the parchment.
Step 2Sift the ground pistachios, almond flour, flour and baking powder into a medium bowl. In a stand mixer fitted with the paddle attachment, cream the butter, sugar, salt and lemon zest on medium-high speed until light and fluffy, about 5 minutes. With the machine running, add the vanilla and then the eggs, one at a time, beating well after each addition. Scrape the sides of the bowl occasionally. Add the pistachio mixture and mix on low speed until just incorporated. Scoop the batter into the prepared pan and spread evenly.
Step 3Bake until golden brown and a cake tester or toothpick inserted in the center of the cake comes out clean, 50 to 55 minutes. Transfer to a wire rack and let cool for 15 minutes in the pan.
Step 4While the cake is cooling, make the glaze.
Step 1In a small saucepan, combine the pistachios, sugar and lemon juice. Place over medium heat and cook, stirring, until the sugar is completely dissolved and the pistachios are nicely coated, about 2 minutes. Remove the pan from the heat.
Step 2Run a thin-bladed knife around the edge of the pan. Place a flat plate on top of the cake and pan. Carefully invert the cake onto the plate by flipping both upside down. Lift the pan off of the cake and discard the parchment paper. Invert your serving plate over the bottom of the cake and flip both so the cake is right-side-up on the plate.
Step 3Immediately spoon the glaze and nuts over the top. Let stand for at least 10 minutes to allow the glaze to set before serving.