If you are using frozen rhubarb, spread it out on the counter for about 5 minutes until it is thawed enough to finely dice.
While rhubarb is thawing, heat oven to 375 degrees F. Grease muffin cups or use paper liners.
Mix flour, baking powder, salt and cinnamon in a bowl. In another bowl, mix egg, brown sugar and jelly until smooth (jelly will not completely dissolve). Add milk, butter and vanilla. Stir in rhubarb. Pour over dry ingredients and mix just until blended.
Scoop batter into muffin cups and bake 20 to 30 minutes or until light brown and springy to the touch in the center.