Preheat your oven to 350°F. Grease and line a standard 9-inch x 5-inch loaf pan and set it aside.
In a large bowl, place the flour, xanthan gum, cornstarch, baking powder, baking soda, salt, poppy seeds, lemon zest, and granulated sugar, and whisk to combine well, breaking up any clumps of lemon zest.
Create a well in the center of the dry ingredients and add the butter, sour cream, eggs, vanilla and lemon juice, and mix to combine.
The batter will be thick, so be sure to press down on the wet ingredients with the back of the mixing spoon to moisten the dry ingredients as you go.
Transfer the batter into the prepared loaf pan, and spread into an even layer with a wet spatula.
For a more even loaf, with a wet, very sharp knife, slash the loaf down the center about 1/4-inch deep.
Place in the center of the preheated oven and bake for 30 minutes.
Reduce the oven temperature to 325°F and continue to bake until a toothpick inserted in the center comes out mostly clean and the top is lightly golden brown (another 15 to 20 minutes).
Remove the loaf pan from the oven and allow the bread to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.
Slice thickly and serve.
Video
Notes
Originally published on the blog in 2014. In 2020, recipe unchanged; video and some photos new; text mostly new.Nutrition information.Nutrition information is per slice assuming a loaf cut into 8 slices, and is an estimate only from online calculators, provided as a courtesy, and should not be relied on under any circumstances.