Preheat oven to 300°F. Line 4 baking trays with parchment paper. Combine flour, rice flour, sugar, and salt in medium mixing bowl. Use your fingertips to rub butter through dry ingredients until mixture resembles fine breadcrumbs. Turn onto a lightly floured bench and knead until smooth. Roll dough between 2 sheets of parchment paper until 1⁄4” thick. Use a 4” pine tree cookie cutter to cut out 24 shapes. Place onto prepared trays, allowing a little room between each cookie. Bake for 40–45 minutes or until shortbreads start to change color. Remove from oven and allow to cool completely on trays. Spread each cookie with icing and sprinkle with nonpareils whilst icing is still wet. For the icing, place egg white into a small mixing bowl; use a fork to beat until foamy. Gradually beat in icing sugar with a wooden spoon until icing is smooth and thick. Cover with a damp tea towel until ready to use.