Preheat oven to 425F.
Remove the leaves and core the cauliflower. Break or cut into bite-sized florets. Scatter onto a baking sheet. Drizzle with olive oil and season with salt and pepper. Stir or toss to coat the cauliflower in oil. Roast in preheated oven for 20-25 minutes, or until cauliflower is tender (test it with a knife) and golden in spots. Remove from oven and set aside.
While cauliflower is roasting, melt butter in a large Dutch oven or soup pot over medium heat on the stove-top. Add the onion and cook, stirring regularly, until softened, but not browned. While the onion is cooking, measure out all the spices and stir together in a small bowl. Set aside.
Add the garlic and ginger and cook, stirring, about 1 minute more. Add the tomato paste and spices and cook, stirring, another minute. Add the canned tomatoes, with their juices, the brown sugar, if using and the chicken broth. Use the edge of a spoon to break up the tomatoes in the pot. Stir to combine. Add the roasted cauliflower to the pot. Bring mixture to a boil, then reduce heat to medium-low and allow so gently simmer for 10-15 minutes, to allow the flavours to blend. (Remember, the cauliflower is already cooked, so you don't need to simmer long).
At this point, I like to give my soup 2 or 3 pulses with an immersion blender. Not too much, as I like to keep some of the cauliflower chunky. If you don't have an immersion blender, a potato masher would probably work, too. Totally optional though, as you like.
Taste soup and add salt and freshly ground pepper, as needed. (Salt generously, to really bring out all the great flavours). Stir in the 1/4 cup of fresh chopped cilantro. Serve warm topped with a dollop of plain yogurt and more chopped cilantro. Add some jalapeno slices if you'd like a pop of heat. Nice served with warm Naan bread.
The cayenne will bring some heat to the pot. You can easily omit for no heat or add as little or as much as you like, to taste. If you happen to have Kashmiri Chili Powder on hand, that's the best option. Add to taste as well.
Don't be shy with salting your soup. I like to save most of the salt until the end of cooking, when I taste and add as needed. If you taste your soup and think "it needs something", it's probably salt!