Preheat oven to 350°F | 177°C
Add to a blender or food processor the shelled pistachio and pulsate until it has a meal consistency.
Grease loaf pan with butter.
Line the loaf pan with parchment paper. Grease again.
Add to first bowl: Gluten Free flour, Pistachio Meal, Cardamom, Baking Powder, Xanthan Guam, and Salt. Set aside.
In a second bowl add: Sugar, lemon zest and softened butter.
Mix the sugar, lemon zest and soften butter very well.
To this second bowl add: 3 Room temperature eggs.
Incorporate the eggs well into the mixture.
Now add the sour cream and vanilla extract.
Incorporate the sour cream and vanilla extract into the mixture.
Add the dry ingredients from the first bowl to the wet ingredients in the second bowl.
Mix the wet and dry ingredients very well.
Pour into the loaf pan. Shake from sided to side to distribute the mix evenly. Pound flat on surface to settle as well.
Place on the top rack of the preheated oven and bake for around 60 minutes. It's useful to confirm the temperature of your oven. I have a thermometer I bought for this.
Check on the loaf and and stick a toothpick or slim knife in the middle. If it comes out clean, with no solids, then it's done.
Let it cool for 15 minutes in the loaf pan.
Leave loaf whole and slice only what you will consume -- that helps it stay fresher longer. Enjoy!