Combine eggs, extract and 1/4 of the sour cream in a small bowl.
Cut butter into chunks.
Put remaining sour cream in another small bowl with tablespoon of oil.
Allow all ingredients to come to room temperature while you prepare a 9-inch springform pan, line bottom with parchment, grease and flour.
You can use a 9x2-inch round pan as well, but it will be a bit more difficult to unmold the cake.
Toast pistachios in a 350 degree (f.) oven for 7-8 minutes, cool then grind fine, (I use a bag and rolling pin)- if using dry measuring cups be sure to grind first then measure Sift all dry ingredients into the work bowl of your mixer and mix on low speed for 30 seconds to combine.
Add softened butter and sour cream, mix on low to combine, scrape bowl, beat on medium (med-high for a hand mixer) for 90 seconds.
This will aerate the cake and give it structure.
Scrape the bowl down and in two additions add the egg mixture, beating for 20 seconds after each addition.
Scrape the bowl one last time and give it a couple pulses.
Pour batter into prepared pan and bake at 350 degrees for 40-50 minutes, (rotate pan 180 degrees after 25 minutes) a toothpick inserted in the center will come out clean and the top of the cake will spring back when lightly pressed.
Remove from the oven and cool on a rack for 10 minutes, remove sides of pan and allow to cool to room temperature.
About 2 hours.
It's delicious slightly warm with just a dusting of powdered sugar, topped with sweetened whipped cream or served with pistachio ice cream.