Preheat the oven to 180°C/Gas Mark 4.
Melt the butter in a pan, turn off the heat and add the chocolate. Allow to cool.
In a bowl, mix the brown rice flour, baking powder, cocoa powder and a pinch of sea salt.
In another bowl, mix the eggs with the sugar. Add the dry ingredients a little at a time, then add the melted chocolate and mix thoroughly to combine.
Pour the mixture into a square-lined baking tin and bake in the oven for 25 minutes.