Preheat the oven to 170C/350F (or 150C fan forced).
Roughly break the chocolate in to pieces, add to a heatproof jug (or bowl) and then slowly melt either on a low heat in the microwave for 5 minutes (stir every so often) or over a double boiler set up on the stove.
While the chocolate is melting, add the cacao powder to a mixing bowl with the salt and boil the water. Pour the boiling water over and whisk. Let sit for 5 or so minutes.
Add the oil to the melted chocolate and then add this mixture to the cacao powder and water. Mix well and let it sit for 5 minutes.
Prepare your baking pan with baking/parchment paper.
Once the 5 minutes is up, add the brown sugar and mix well.
Next, add the sweet rice flour and the baking powder and mix well until no flour is visible and there are no lumps. It will be a thick batter.
Lastly, add the room temperature water and mix in very well. Pour the batter in to your prepared baking pan and smooth the top with a spatula.
Put the dish in the oven on the middle rack. If using glass, bake for 25-30 minutes (cover with foil and turn at the halfway mark) or 20-25 minutes if using a metal pan (cover with foil and turn at the halfway mark). The lower amount of time will give you a fudgier centre and the higher cook time gives a larger chewy edge.
Remove from the oven and cool on a rack. Allow to cool completely before cutting - otherwise they may break. You can also speed things up by putting them in the fridge for a while to cool down.
Store in the fridge for up to a week or in the freezer for up to 2 months. Defrost at room temperature. I prefer to eat these straight from the fridge (chewier) or at room temperature. You can heat them on a low power in the microwave for 10-15 seconds if preferred.