1 3/4 cups all purpose gluten free flour
1 teaspoon xanthan gum (omit if your blend already contains it)
6 tablespoons cornstarch
1 1/2 teaspoons baking powder
1/2 teaspoon baking soda
1/2 teaspoon kosher salt
2 tablespoons poppy seeds
Finely grated zest and juice of 1 medium-size lemon (about 3 tablespoons juice)
3/4 cup sugar
6 tablespoons unsalted butter, at room temperature
1 cup sour cream, at room temperature (can replace with an equal amount Greek-style yogurt)
2 eggs at room temperature, beaten
1 teaspoon pure vanilla extract
Preheat your oven to 350°F. Grease 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 and poppy seeds, and whisk to combine well. Add the lemon zest, and whisk to combine, breaking up any lumps. Add the sugar, and whisk again to combine. Create a well in the center of the dry ingredients and add the butter, sour cream, eggs, vanilla and lemon juice, mixing to combine after each addition. The batter will be thick.
Scrape 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 10 to 20 minutes). Remove from the oven and allow to cool in the pan for 15 minutes before transferring to a wire rack to cool completely. Slice thickly and serve.