The white powder on brioche buns is just flour which has been dusted onto the surface of the brioche bun before it has been baked.
The dusting of the flour on the brioche buns before baking gives the brioche buns a crisp and slightly powdery texture which is a common practice in bakery production to prevent sticking and also enhance the appearance of the bread.
The reason why brioche is considered to be unhealthy is because brioche contains more fat, saturated fat and calories due to the excess butter that is in brioche.
Brioche also has higher sugar content than regular bread.
While white bread is not the healthiest it is slightly healthier than brioche due to the lack of saturated fat and lower calories.
Whole grain breads are healthier than both white bread and brioche.
Brioche is not healthier than regular bread as it contains more fat and more calories than regular bread.
However brioche does have a higher protein content than regular plain white bread due to the eggs which are beneficial for health.
Brioche is also high in saturated fat due to the excess butter.
Brioche is healthy in moderation but when eaten in excess it can be unhealthy.