Prepare muffin pans, using paper cups. This makes 16 cupcakes.
Preheat oven to 350℉
In a small bowl, sift or whisk together flour, baking soda, and salt. Set aside.
In the large bowl of a stand mixer, cream together the brown sugar and butter.
Add the beaten egg, and mix until well blended.
Mix in the vanilla.
Alternate adding the milk and flour mixture, about ¼ of the milk, and them ¼ of the flour mixture at a time, mixing well after each addition. Continue until all the milk is added to the batter.
Remove the bowl from the mixer, and mix in the rhubarb by hand with a spatula.
Spoon batter into cupcake/muffin pans.
Sprinkle with sugar/cinnamon topping
Bake in preheated oven, 25 minutes, or until a knife comes out clean.
May be served warm with a scoop of ice cream, or whipped cream.