Line a baking sheet with parchment paper. Preheat the oven to 325℉ (163℃).
In a medium mixing bowl, whisk together the flours, poppyseeds, xanthan gum, baking powder, and salt.
1¼ cup white rice flour, 1 cup almond flour, 1½ tablespoons poppyseeds, 1 teaspoon xanthan gum, 1 teaspoon baking powder, ½ teaspoon fine sea salt
In a large mixing bowl, cream the butter. (Either by hand or with an electric mixer.) Beat in the eggs, lemon juice, zest, and vanilla. You might need to pause to scrape down the sides of the bowl with a spatula to make sure everything is incorporated.
¼ cup butter, ½ cup raw sugar, 3 large eggs, 2 tablespoons lemon juice, 1 tablespoon lemon zest, 1 teaspoon pure vanilla extract
Stir the dry flour mixture into the wet egg mixture.
Press the dough into a long, flatish shape, about 4 inches wide, on the parchment-lined baking sheet.
Bake for 30 minutes.
Remove from the oven. Allow it to cool for 10 minutes. Slice the "log" crosswise into ½-inch thick cookies. Lay them out individually on their sides, and bake for another 25 minutes.
Video
Notes
If you would like to freeze the cookies for later, you can do so after the first baking, cooling, and slicing. Then, when you pull them out of the freezer, bake them the second time, and you'll have nice, freshly baked biscotti!