Preheat oven to 350 F. Line a sheet pan with parchment. Combine brown sugar, oats, flour, and cinnamon in small bowl. Cut in butter with a pastry blender or 2 knives until mixture is crumbly. Sprinkle streusel over the sheet pan and bake 20 minutes, reducing temperature to 325 F. after 15 minutes, if streusel seems to be browning too quickly. Cool to room temperature then store in a container with a tight fitting lid for up to 2 weeks.

In a blender, combine all ingredients together on high for 30 seconds or until smooth. Pour into a 20 oz. glass and top with a sprinkling of streusel.