Preheat oven to 425 degrees.
In a bowl, microwave ½ cup butter or margarine for 10 seconds, remove and stir. Repeat this step until butter is melted. Let cool.Add ¼ cup milk and 1 egg. Stir together thoroughly.
In a second bowl, mix 2 cups flour, 1 cup brown sugar, 1 Tbsp baking powder, 1 tsp baking soda, ½ tsp salt and ½ tsp cinnamon.
Add wet mixture to dry ingredients and blend until just moistened. Add sliced rhubarb and mix gently. Do not over-stir.
Fill lined muffin cups with batter (we used large liners – alternatively you can grease the muffin cups).
Mix 3 Tbsp white sugar and 1 tsp cinnamon and sprinkle overtop the muffins – each muffin should get about ½ tsp of the mixture but you don’t need to be exact.
Bake for 15 to 20 minutes.
Best eaten when still warm!