1. Mix flour, sugar, salt, and crumbled yeast in a large bowl. Whisk together beaten egg, milk, and softened butter, then add to flour mixture. Knead for 10 minutes until soft and smooth. Cover and let rise at room temperature for 2 hours until doubled.

2. While dough rises, combine rhubarb, raspberries, and preserving sugar in a wide pan. Bring to boil, stirring constantly. Boil for 4 minutes until thickened. Cool slightly, puree until smooth, then cool completely.

3. Divide risen dough into 8 equal portions and shape into smooth balls. Place on parchment-lined baking sheet with space between. Cover and let rise for 30 minutes until puffy.

4. Heat oil in heavy pot to 320°F (160°C). Flatten each dough ball slightly by hand. Using slotted spoon, slide 2-3 doughnuts into hot oil bottom-side up. Fry for 4 minutes per side until deep golden brown. Drain on paper towels.

5. Cool doughnuts for 10 minutes. Insert straw two-thirds into each doughnut, wiggling to create 5mm opening. Transfer jam to piping bag with 5mm smooth tip. Pipe jam into each doughnut. Dust generously with powdered sugar.