Asome times my buttercream is perfect and i love it . its the best one ever. I dont feel any grains from the powder sugar and its very smooth. But there is times when the butter separate on the cake and i could see tiny yellow lines on the buttercream and it start to look sloppy. I hate when that happens and i dont know why it happens. the weather in my kitchen is good. my recipe is to add hot water to the powder sugar mixing them and then start adding the butter. Please could some one enlight me what i am doing wrong.

I'd use milk or cream instead of water. Start by creaming the butter, then add the liquid until it mixes in completely to the butter. Then add the confectioner's sugar a little bit at a time until it mixes in together. Scrape the bowl to make sure you get all the butter off the bowl while you'r emixing in the sugar.