I prefer to use a food processor for this recipe because it breaks the oats down a bit and makes the mixture more consistent. However, making this topping by hand is super easy and usually results in a more textured topping because the oats don't break down as much. This is equally as delicious but it does tend to brown more in the oven because of the larger crumbles. When I do this by hand I usually use room temperature butter or you can even use melted butter.