If your batter is solidified and hard to squeeze,
①you should cream the butter until it becomes soft mayonnaise-like.
②before you add flours, you should take the batter from the hand mixer and put it in the original batter.
③after you add flours and fold, let the batter sit at room temperature for 5-10 mins and then mix it again to smooth and soft.

Preheat a oven to 392 °F. Sift powdered sugar. Make unsalted butter smooth. Cream the butter until it becomes soft mayonnaise like. Add the powdered sugar in 3 parts. Mix well each time.

Mix it with a hand mixer on low speed. Mix until it becomes white.

Add egg yolk. Mix it evenly on low speed.

Add milk and vanilla oil and mix well on low speed. (You should take the batter from the hand mixer and put it in the original batter.)

Sift cake flour. Add all the flour to the batter at once. Fold it.

Fold it until the lumps are formed. Mix while pressing for 30-40 sec to make smooth. If your batter is solidify and hard to squeeze, let the batter sit at room temperature for 5-10 mins and then mix it again to smooth and soft.

Put it in a piping bag. (star piping tip) Squeeze out a small amount　and draw a circle twice.

Bake it at 338 °F for 17 minutes. Put strawberry in the middle dent. Bake it at 338 °F for 3 minutes again.

Cool it well. It's all done!

(star piping tip I used)