Preheat oven to 240 C or hottest setting. Melt the butter and leave to cool. Dip a pastry brush in the melted butter and lightly grease a 22 cm square tin. Combine the gluten free flour, corn meal, salt, sugar and baking powder in a large bowl. Make a well in the centre. Add the eggs and the milk and yoghurt. With a fork, stir to combine. Add the melted butter and mix through gently until just combined. Pour the mixture into the prepared tin. Bake for 15 minutes or until a wooden skewer inserted in the centre comes out clean. Leave to cool in the tin for a few minutes then cut into squares and serve.
Note: The type or brand of polenta used can give you very different batters. If the batter seems very thick and reluctant to pour out of the bowl, add a little more milk. If the batter seems very liquidy, add a little more polenta. The ideal consistency should be like a thick cake batter