You will need two 12 hole Muffin tins or one 12 hole and one 6 hole tins, greased or paper-lined
In a bowl whisk together brown sugar, oil, eggs, and vanilla; mix well. Add buttermilk, rhubarb and walnuts or pecans.
In another bowl combine flour, baking powder, baking soda, and salt.
In another bowl combine sugar, cinnamon, and butter.
Add the wet mixture to the flour mixture and gently fold together, do not over mix or your muffins will be tough.
Spoon batter into prepared muffin tin, filling three-quarters full. Sprinkle with topping and add one whole pecan nut to the top of each muffin.
Bake in preheated oven for about 20 minutes.
This recipe calls for buttermilk if you don't have any you can make an easy substitute. Take 250ml of milk and add 1 tsp of either lemon juice or white vinegar, stir together and leave for a few minutes before using.