Using an electric mixer and a large mixer bowl, cream together the butter and sugar on medium speed until light and fluffy. Mix in the eggs on low speed until blended. Add the grated lemon peel.

In a medium bowl, sift together the flours, xanthan gum, baking powder, baking soda, and salt.

In another medium bowl, combine the buttermilk and vanilla. On low speed, beat the dry ingredients into the butter mixture, alternating with the buttermilk -- beginning and ending with the dry ingredients. Mix just until combined. Spoon the batter into the prepared pan and smooth the top.

Bake the loaf pan for about 50 minutes, cake pans for 25-30 minutes or until top is golden brown and tester inserted into center comes out clean. Cool cake in pan(s) for 5 minutes, then remove from pan(s) and cool on rack.