The pudding mixture will look slightly lumpy until you stir in the butter. The butter will help create emulsification and make your pudding nice and smooth. Another reason your pudding will turn out lumpy is if you pour all of the hot milk into the cornstarch mixture quickly without stirring. Once the hot liquid hits the corn starch, it will instantly start to set, and you will get lumps. This is why it’s important to keep whisking and slowly pour the hot liquid into the cornstarch mixture.