Preheat oven to 350 degrees. Combine brown sugar, oil, egg, vanilla and buttermilk; mix until well blended. Add flour, baking soda, baking powder and salt, mix well. Add rhubarb to mixture and combine. Use a heaping tablespoon for each muffin. Mix 1/3 cup sugar and 1 tsp cinnamon in melted butter until blended and place a tsp full on top of each muffin before baking.
In a bowl, combine the flour and sugar; cut in butter until the mixture resembles coarse crumbs. Press into a greased 13 x 9 baking pan. Bake at 350 degrees for 10 minutes. While crust is baking, for filling, combine sugar, flour cream and eggs until well blended. Stir in the rhubarb and pour over crust. Bake at 350 degrees for 40-45 minutes or until custard is set. Cool. For topping, beat cream cheese, sugar and vanilla until smooth; fold in whipped cream. Spread over top. Cover and chill. Store in refrigerator.