01 - Whisk together flour, granulated sugar, yeast, and salt in a large bowl until thoroughly combined.
02 - Add lukewarm milk, room temperature eggs, and softened butter to the flour mixture. Mix until a sticky, shaggy dough forms.
03 - Knead dough on a lightly floured surface for 8–10 minutes until smooth and elastic, or use a stand mixer with dough hook for 6–7 minutes on medium speed.
04 - Place dough in a greased bowl, cover with plastic wrap or damp towel, 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 dough rounds on a parchment-lined baking sheet, cover loosely, and let rise for 30–40 minutes until puffy.
07 - Heat vegetable oil in a deep fryer or heavy-bottomed pot to 350°F, maintaining temperature throughout frying.
08 - Fry doughnuts in batches for 1–2 minutes per side until golden brown. Transfer to paper towels to drain excess oil.
09 - Roll warm doughnuts in granulated sugar while slightly warm to ensure even coating. Allow to cool completely before filling.
10 - Combine diced strawberries, 2 tablespoons sugar, and lemon juice in a saucepan. Cook over medium heat for 3–4 minutes until soft and juicy. Mash lightly with a fork and cool completely.
11 - Whip chilled heavy cream with powdered sugar and vanilla extract until stiff peaks form. Gently fold in mascarpone cheese and cooled strawberry mixture until smooth and well incorporated.
12 - Transfer filling to a piping bag fitted with a round tip. Cut a small slit in the side of each cooled doughnut and pipe strawberry cream generously into the center.