Preheat oven to 350°F | 177°C
Grease loaf pan with butter and line with parchment paper. Grease again.
Add to first bowl: Gluten Free flour, Pistachio Meal, Cardamom, Baking Powder, Xanthan Guam, and Salt. Set aside.
Second bowl add: Sugar, lemon zest and softened butter. Mix well.
Second bowl add: 3 Eggs and mix well.
Second bowl add: Sour cream and vanilla extract. Mix well.
Add first bowl with the dry ingredients to the second bowl with wet ingredients and mix 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 pre-heated 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!