01 - Whisk together flour, sugar, yeast, and salt in a large bowl until thoroughly combined.
02 - Pour lukewarm milk into the dry mixture. Add eggs and softened butter. 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 speed.
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 volume, approximately 1 to 1.5 hours.
05 - Punch down risen dough and roll out to 1/2-inch thickness. Cut rounds using a 3-inch cutter or doughnut cutter.
06 - Arrange rounds on a parchment-lined baking sheet, cover loosely, and let rise for 30-40 minutes until puffy.
07 - Pour vegetable oil into a deep fryer or heavy-bottomed pot. Heat 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 doughnuts are still warm, roll generously in granulated sugar until evenly coated. Allow to cool completely on a wire rack.
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 strawberries and let cool completely.
11 - Whip chilled heavy cream with powdered sugar and vanilla extract until stiff peaks form. Gently fold in mascarpone cheese until smooth, then incorporate cooled strawberry mixture.
12 - Transfer strawberry cream to a piping bag fitted with a round tip. Cut a small slit in the side of each cooled doughnut and pipe filling generously into the center.