01 - Whisk together flour, sugar, yeast, and salt in a large bowl until evenly combined.
02 - Add lukewarm milk, eggs, and softened butter to the dry ingredients. Mix until a sticky dough forms.
03 - Knead the dough on a lightly floured surface for 8–10 minutes until smooth and elastic, or use a stand mixer with a dough hook for 6–7 minutes.
04 - Place the dough in a greased bowl, cover, and let rise in a warm place until doubled in size, about 1–1.5 hours.
05 - Punch down the dough and roll out to 1/2-inch thickness. Cut out 3-inch rounds using a cutter.
06 - Place rounds on a parchment-lined tray, cover, and let rise for 30–40 minutes.
07 - Heat vegetable oil in a deep fryer or large pot to 350°F.
08 - Fry doughnuts in batches for 1–2 minutes per side or until golden brown. Drain on paper towels.
09 - While still warm, roll doughnuts in granulated sugar to coat. Cool completely.
10 - Combine strawberries, 2 tbsp sugar, and lemon juice in a saucepan. Cook over medium heat for 3–4 minutes until soft. Mash lightly and cool.
11 - Whip heavy cream with powdered sugar and vanilla until stiff peaks form. Gently fold in mascarpone and cooled strawberry mixture until smooth.
12 - Transfer filling to a piping bag fitted with a round nozzle. Cut a small slit in the side of each doughnut and pipe strawberry cream generously into the center.