Recently I've tried making nutella buttercream, however it always becomes curdled and grainy. I have no idea what is wrong with it. I'm using the follow ingredients :

100g butter, 150g nutella, 225g sifted icing sugar,  3 table spoon whole milk. 

I usually cream the butter and nutella together for 5minutes before adding the icing sugar in batches. After adding all the icing sugar, it started to become very hard so I added a tablespoon of milk at a time. The end product became very grainy and curdled. 

I tried doing the same method with different amount of ingredients. 

130g butter, 200g nutella, 130g icing sugar , 1table spoon whole milk. 

This time it came out better as its more firm. However I could see that the buttercream is greasy and it looks a bit curdled.

Please advise me on what to do. I'm using a stand mixer with a paddle attachment. 

My thought is to ditch the milk altogether? Could you add coffee instead (it should just enhance the chocolate flavor if you don’t add too much) or even water to thin it out a bit and see if the outcome is better and doesn’t curdle?