Melt the butter and oil in a large frying pan. Chop the onions, bell peppers, carrot and garlic (if doesn't need to be fine as they are going to be blitzed in a food processor) and cook slowly for about 20 minutes until they are soft. Don't skimp on this phase as the slowly cooked vegetables (especially the onions) really make the taste of this curry.
Mix the spices together and add to the vegetables, cooking for around a minute or so. Add the tinned tomatoes and then fill the empty tin with the same amount of water and add that in, too. Allow the mixture to bubble and then lastly, pour in the tin of coconut milk. Stir until combined and then allow the mixture to come to the boil once more.
Remove from the heat and pour into a food processor. Blitz until you have a thick sauce (it doesn't matter if there are some larger pieces). TAKE CARE! The sauce is hot and may spurt over the top if you add too much. Do it in batches if you need to. Pour back into the pan you were using before.
Chop the chicken in fairly small pieces and place in the curry sauce. I like to prepare my curry in advance and leave it at this point for an hour or two, to allow the hot sauce to soak into the chicken, but you can just carry on with the next stage immediately if you prefer.
Cook the chicken for around 20 minutes (don't overcook but make sure it has been cooked right through).
For the yellow rice: use as much as you need. In a large pan, heat the amount of water specified on the rice packet along with one tablespoon of turmeric. Add the rice and cook as per instructions. Serve immediately with the curry. Enjoy!