Heat the oven to 150°C/Gas 2. Combine the butter, icing sugar and sea salt in a food processor and whiz until smooth.
Sift in the flour and rice flour, then pulse off and on, scraping down the sides from time to time, until the mixture gathers into a ball. Knead for a minute or two until smooth, then cut into two, wrap in cling film and refrigerate for 30 minutes.
Turn onto a fl