Preheat oven to 180 C / 350 F. Sift together flours, baking soda and salt until well combined. Cream butter, vanilla and sugar in the food processor until smooth and creamy. Add in the dry ingredients and lemon zest and pulse until the mixture forms a workable dough. If batter is too sticky, just add a pinch more rice flour a little at a time until workable. Roll out between two pieces of parchment paper to about 2 mm / ¼ inch thick. Cut with cookie cutters and place on a lined baking sheet. Bake for 15 minutes - the cookies will yield to the touch slightly and will not be completely hard. Take out of the oven and leave on the tray for a couple of minutes before placing on a rack to cook and solidify.