To make pistachio flour, take shelled, unsalted, roasted pistachios (buy them unshelled and save yourself some time) and pulse in 1/4 of the pistachios in the food processor just until beginning to break down. Pass through a sieve to get the flour and return the pistachio pieces back to the food processor. Repeat until a good amount of the pistachios are flour (you will have meal left over, use it to top the cake.) Just be careful not to over pulse the nuts and turn them into butter- patience is key. If you try to make the cake with pistachio meal, the texture won't be the same!
Find it online: https://naturallyella.com/pistachio-olive-oil-cake/