01 - Whisk together flour, granulated 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 to 10 minutes until smooth and elastic, or use a stand mixer with dough hook for 6 to 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 to 1.5 hours.
05 - Punch down risen dough and roll out 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 to 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 to 2 minutes per side until golden brown. Transfer to paper towels to drain excess oil.
09 - Roll warm doughnuts in granulated sugar while still slightly warm, then cool completely on a wire rack.
10 - Combine strawberries, 2 tablespoons sugar, and lemon juice in a saucepan. Cook over medium heat for 3 to 4 minutes until softened. Mash lightly with a fork and cool completely.
11 - Whip heavy cream with powdered sugar and vanilla extract until stiff peaks form. Gently fold in mascarpone and cooled strawberry mixture until smooth and uniform.
12 - Transfer filling to a piping bag fitted with a round nozzle. Cut a small slit in the side of each cooled doughnut and pipe strawberry cream generously into the center.