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Combine flour, sugar, and salt in a large bowl. Add egg, lukewarm milk, and yeast. Mix until it becomes a dough. Cover the bowl and let sit for 15 minutes.

Mix in the softened butter. Transfer onto a worksurface (lightly flour it if your dough is too sticky to handle). Knead until it becomes smooth and not sticky.

Shape into a dough ball. Grease it with some vegetable oil and place in a large bowl. Cover the bowl and let sit for about 1 hour.

Punch down the dough and divide it evenly into 10 equal pieces. Take 1 piece and shape it into a ball. Press down to flatten it a little. Repeat with the rest. Cover and let them rest for 30 minutes.

Heat at least 2-inches (5 cm) of vegetable oil in a deep pot until it gets to 320°F (160°C).

Place one dough at a time into the oil. Fry the one side until golden brown, 2-3 minutes. Then flip and fry for another 2-3 minutes. Remove from the oil onto a wire rack lined with paper towel.

Before the donuts cool completely, dip them into a bowl of granulated sugar to coat the entire surface.

Poke a hole into each donut with a chopstick or paring knife. Then fill each with enough strawberry jam using a piping bag. Serve and enjoy!