01 - Whisk together flour, sugar, yeast, and salt in a large mixing bowl until thoroughly combined.
02 - Add lukewarm milk, eggs, and softened butter to the dry ingredients. Mix until a sticky, shaggy dough forms.
03 - Knead 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.
04 - Place dough in a greased bowl, cover with plastic wrap or towel, and let rise in a warm, draft-free area until doubled in size, 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, and let rise for 30-40 minutes until puffy.
07 - Heat vegetable oil in a deep fryer or heavy-bottomed pot to 350°F.
08 - Fry doughnuts in batches for 1-2 minutes per side until golden brown. Remove with slotted spoon and drain on paper towels.
09 - While still warm, roll each doughnut in granulated sugar to coat evenly. Transfer to wire rack and 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 let 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 combined.
12 - Transfer filling to piping bag fitted with round tip. Cut small slit in side of each cooled doughnut and pipe strawberry cream generously into center.