Nut flours — nuts ground to a flour-like powder — are popular with bakers because they add bulk to a recipe like wheat flour but add their own flavor too. They’re also a cornerstone of gluten-free baking.
Zoe Nathan’s Lemon-Pistachio Cake calls for almond flour, which is commonly available from grocery stores where they might carry brands like Bob’s Red Mill, and finely ground pistachios, which is not a nut flour you can usually buy in stores. But it’s easy to make nut flours yourself at home.
First, toast the nuts — toasting nuts develops their flavor and makes for a tastier finished product. Nathan’s recipe calls for 1 cup of ground pistachios, so start with 1 1/2 cups of whole, shelled nuts, since it’s better to have a little left over than to have to start all over again because you’re short a couple tablespoons. Spread the pistachios on a rimmed baking sheet and bake in a 325-degree oven until the pistachios smell nutty and are lightly browned, 8 to 10 minutes.
Allow the pistachios to cool completely on the baking sheet before grinding them or else you’ll end up with pistachio butter, not flour. Transfer the nuts to a food processor and pulse until finely ground, which can take about 1 minute total. If you see any pistachio bits starting to clump together around the bottom edge of the processor bowl, stop pulsing and scrape and stir the nuts to evenly distribute them before continuing. Once ground, pass the flour through a medium-mesh sieve to strain out any large pieces. Transfer any large pieces back to the food processor and pulse until finely ground.
Measure and use the ground pistachios right away for the cake or transfer them to a zip-top plastic bag and store in the refrigerator or freezer since the oils in pistachios and other ground nuts can go rancid quickly at room temperature.
When baking, if you’re not using a scale, be sure to pack the pistachio flour into the cup like you would for brown sugar, not to scoop and level like you would for regular flour. In my experience with baking recipes that call for ground nuts, this is the method used to measure the ingredient for volume. It will ensure your cake comes out with the proper and intended texture.