1. In a medium bowl, mix flour and Bread Booster together.
2. In the bowl of a stand mixer, combine 2 cups of flour mixture, sugar, dry yeast and salt. In a microwave-safe bowl add water and butter. Microwave on HIGH in 15 second increments until very warm (120° to 130°F. Butter won’t melt completely). Add to dry ingredients with the egg.
3. In a stand mixer on medium speed, beat for two minutes, scraping bowl occasionally. Add 1 cup flour mixture; beat 2 minutes at high speed, scraping bowl as needed. Stir in just enough remaining flour mixture so that the dough will form into a ball.
4. Knead on lightly floured surface until smooth and elastic and dough springs back when lightly pressed with 2 fingers, about 6 to 8 minutes. Cover with a towel; let rest for 10 minutes.
5. For filling, combine sugar and cinnamon in a small bowl. Set aside.
6. Roll dough into a 15 x 10-inch rectangle using a rolling pin. Spread 2 tablespoons butter over dough stopping at least 1/2-inch from the edges on the long sides. Sprinkle with cinnamon sugar mixture. Toss rhubarb and strawberries with corn starch and sprinkle over sugar. Beginning at long end of each rectangle, roll up tightly. Pinch seams to seal. Cut into 12 equal pieces. Place, cut sides down, in greased 13 x 9 inch baking pan. Cover with towel; let rise in warm place until doubled in size, about 30 to 40 minutes.
7. Bake in preheated 350º F oven for 25 to 30 minutes or until rolls are golden brown. Cool on wire rack for at least 20 minutes.
8. Combine all frosting ingredients in a large bowl with mixer and beat until creamy. Spread over cooled rolls.

*Do not substitute frozen fruit.