1. Mix flour, sugar, salt, and crumbled yeast in large bowl. Combine beaten egg, milk, and soft butter in separate bowl. Add wet ingredients to flour mixture and knead about 10 minutes until soft and smooth.

2. Cover bowl with kitchen towel. Let dough rise at room temperature about 2 hours until doubled in size. Don’t rush this step – patience creates fluffy doughnuts!

3. While dough rises, make rhubarb jam. Combine rhubarb pieces, raspberries, and preserving sugar in wide pan. Bring to boil, then boil 4 minutes while stirring constantly. Let cool slightly, puree with immersion blender, and cool completely.

4. Divide risen dough into 8 equal portions. Shape each into smooth ball and place on parchment paper. Cover and let rise 30 minutes more until puffy.

5. Heat oil in deep pot to 160°C (320°F). Use candy thermometer to check temperature. Flatten dough balls slightly by hand before frying.

6. Using slotted spoon, carefully slide dough balls into hot oil bottom-side up. Fry 4 minutes on each side until deep golden brown. Drain on paper towels.

7. Once cool enough to handle, insert drinking straw two-thirds into each doughnut. Wiggle straw back and forth to create 5mm hole. Transfer cooled jam to piping bag with smooth 5mm tip and pipe into doughnuts. Dust generously with powdered sugar.