Prepare 6 sheets of parchment paper 10 cm / 4'' square. Draw a 6.5 cm / 2.5'' circle on papers. Beat eggs lightly; set aside. Please use room-temperature eggs to prevent choux pastry from getting cold. Mix cake flour and bread flour and stir with a whisk to crush lumps; set aside.

Put water, butter, sugar, and salt in a small pot. Melt the butter over low heat. Turn up the heat to medium or high heat and bring it to a boil until bubbling up, and turn off the heat.

Add flour, and use a spatula to mix until smooth.

Heat it again over medium heat for about two and a half minutes while kneading it, and quickly put it in a large bowl.

Let deep-frying oil warm over low heat in advance. (Deep-fry the dough while it's warm to make the donuts fluffy.) Add half the beaten egg to the dough, and mix well until smooth.

Add the remaining egg little by little (1 tbsp) and mix until smooth each time. Mix it until stiff peaks form. You don't have to use up the egg. If stiff peaks form when you scoop it, that's just right, not too hard and not too soft.

Transfer it into a piping bag fitted with a star tip. I used a 6-cuts star tip, a 1.5 cm (0.6'') tip diameter. Stack two piping bags because the dough is hard. Put the paper with the circle facing the backside. Squeeze out the dough along the circle. Gently press the end to stick.

Deep-fry them at 160 ℃ / 320 F. First, put each in the oil with the paper side facing up. Deep-fry it while holding the paper for about 10 seconds, and then let go. Deep-fry for 2 minutes as it is.

Turn it over and gently peel off the paper. Deep-fry for another 5-8 minutes, turning it over occasionally until light golden brown. Drain excess oil. Let it cool well.

[ For Cream ]
Add granulated sugar to the whipping cream. Whip it until heavy while cooling in ice water, add vanilla essence, and mix it. Take it out from the ice water and whip it until soft peaks form.

Cut each cruller donut in half and put the whipped cream between the slices of donuts. Sprinkle powdered sugar, and it's all done! The powdered sugar dissolves into donuts as time passes, so it’s better to sprinkle it just before eating.