Remove butter and lemon from fridge to ensure they are room temperature.
Rinse and chop rhubarb into small pieces. If stalks are thick, slice lengthwise first. Add dried cranberries and 2 T sugar.
Roll lemon on counter to maximize amount of juice. Cut in half and save one half for another use. Zest skin, being careful not to zest too deeply into the white part. Squeeze as much juice as possible, aiming for ¼ cup. Add lemon juice and zest to rhubarb mixture, stir and allow to stand for 30 minutes.
Make streusel topping by melting butter, then stirring in the rest of the ingredients. Preheat oven to 350° and ensure rack is set at centre of the oven.
Sift together both types of flour, baking soda, baking powder and salt.
Using hand mixer, cream butter. Add brown and white sugar and blend well. Mixture will be lumpy. Add yogurt, egg and vanilla and mix on low speed. Add half of flour mixture and stir with spoon. Add half of rhubarb and mix gently. Add other half of each mixture, stirring gently just until combined. Batter will be very stiff.
Scoop batter into paper lined or greased muffin tins. Use larger tulip shaped paper muffin cups for 12 muffins. Sprinkle streusel topping evenly over muffins.
Bake for 25 - 30 minutes until a toothpick inserted comes out clean. If desired, set oven to convection bake for last 5 minutes to help brown the streusel topping.If making smaller muffins, bake for 20-22 minutes. Cool at least 30 minutes.