My milk chocolate is grainy - how to fix it?
Sylvia-

Hi! The graininess may be caused by either ingredients or method. When making chocolate, I choose to use cocoa butter as opposed to other fats. It marries with the cocoa powder in a way that other ones seem to have problems with. Furthermore, I choose to combine the cocoa butter and cocoa powder with a violent whisk prior to the addition of heat, to ensure that it is mixed very well before cooking together. Once the consistency is correct through cooking over the boiling water, I remove from the heat and add the sweeteners and milk, also adding salt to taste. Removing from heat isn’t necessarily required, but I find that it is less likely to roast on me.

Another note is that it would do well to give your course ingredients a whirl in a grinder or food processor. I use granulated sugar, or other times, unprocessed sugar. Both of these course ingredients get a good amount of time with blades to create a finer product. Seeing as you are using powdered sugar, this shouldn’t be a problem - unless it becomes lumpy.