Heat the plant-based butter in a large nonstick skillet over medium heat. When the butter has melted, stir in the curry powder. Mix the curry thoroughly with the butter, stirring for the next 2 minutes or so. Add the flour, continuing to stir, until the flour is lightly toasted (the color of the mixture will be slightly darker), another 2-3 minutes.
Add the dashi and 2 cups of the vegetable stock. Stir very thoroughly with a pair of chopsticks or non-scratching whisk, until the consistency is smooth.
When the mixture returns to a boil, add the soy sauce, mirin, nutritional yeast, and plant milk (if using). Stir again.
Cover, turn to low heat, and let simmer for 5 minutes. If you need to keep the broth warm for longer and the broth thickens, you can stir in some of the reserved vegetable stock to thin it out.