In crumble recipes, cold butter is cut into sugar and flour to create a dry, powdery streusel topping that melts into a crumbly, only slightly crispy blanket when baked. When the fruit releases juices, much of the powdery topping is washed into the filling. To create a sturdier topping melted butter is incorporated directly into the flour, oatmeal and nuts to bind all the ingredients together so it bakes as a separate entity and doesn’t wash away.  Bonus, this method is super easy!