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 mixture. Mix until a sticky, shaggy dough forms.
03 - Knead on a lightly floured surface for 8–10 minutes until smooth and elastic. Alternatively, use a stand mixer with dough hook for 6–7 minutes on medium-low speed.
04 - Place dough in a greased bowl, cover with plastic wrap or a damp towel, and let rise in a warm, draft-free area until doubled in volume, approximately 1–1.5 hours.
05 - Punch down risen dough and roll to 1/2-inch thickness. Cut 3-inch rounds using a doughnut cutter or round cookie cutter.
06 - Arrange rounds on a parchment-lined baking sheet, cover loosely, and let rise for 30–40 minutes until puffy and noticeably increased in size.
07 - Heat vegetable oil in a deep fryer or heavy-bottomed pot to 350°F. Maintain temperature throughout frying.
08 - Fry doughnuts in batches, cooking 1–2 minutes per side until golden brown. Remove with a slotted spoon and drain on paper towels.
09 - Roll warm doughnuts in granulated sugar while still slightly warm to ensure even coating. 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 softened. Lightly mash and cool completely.
11 - Whip chilled heavy cream with powdered sugar and vanilla extract until stiff peaks form. Gently fold in mascarpone and cooled strawberry mixture until smooth and well combined.
12 - Transfer filling to a piping bag fitted with a round tip. Cut a small slit in the side of each doughnut and pipe strawberry cream generously into the center until filled.