In a large bowl add all of the streusel ingredients EXCEPT the butter and mix to combine. Add in the very soft butter and with either clean hands or a fork, press the butter into the oat mixture until it forms clumps and coated crumbs.
Use it on your favorite fruit dessert, cookie, quick bread, or muffins.
To bake as pieces, preheat the oven to 325F and line a rimmed baking pan with parchment. Place the streusel in a single layer, and bake until golden brown or about 25 minutes. Let it cool completely in the sheet pan on a wire rack. Once cooled, break it up into small pieces. Store in an air-tight container for up to a week.
Notes
This can be made 1 day in advance. Simply make, place in an air-tight container, and refrigerate for up to 3 days.