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