Preheat oven to 350°F.
In a medium-sized mixing bowl, beat sugar and 1/2 cup butter until fully combined.
Mix in cake flour and baking soda.
Mix in eggs one at a time.
Mix in orange juice.
Fold in strawberries and rhubarb.
Fill cupcake liners 3/4 full.
Bake for 20 minutes or until cupcakes bounce back when lightly touched.
In a medium-sized mixing bowl, mix 1 cup butter until light and fluffy (about three minutes on high speed).
Mix in powdered sugar a little bit at a time.
Mix in vanilla bean paste and lemon zest.
Spread or pipe onto cooled cupcakes.