Dice chocolate into about 1 cm (0.4 in) cubes. Sift cake flour and baking powder together twice. Preheat an oven to 200℃ / 392 F. Put unsalted butter in a large bowl. Cream the butter until soft.

Add granulated sugar in 3 parts. Mix well with a spatula each time until combined. Mix it with a whisk until it becomes white and fluffy (for 5 mins).

Beat an egg lightly until it gets watery and add it to the butter in 4 parts. Mix thoroughly after each addition until creamy. (Please use an egg of room temperature to prevent the batter from separating.)

Put pistachio flour into the bowl while sifting. Mix it until combined.

Add half of the milk and mix until almost combined. Put all the flour you sifted and fold 10 times. Add the remaining milk and fold it 10-15 times.

Scrape the flour on the inner side of the bowl. Fold the batter 15 times until smooth and a little glossy. Transfer the batter to a piping bag.

Line muffin mold with glassine paper. Pipe the cake batter into each cup. Drop the mold lightly 4-5 times.

Sprinkle the chocolate cubes on each muffin. The muffins rise when baked, so you should put the chocolate around the middle. Push them in lightly. Bake it at 180℃ / 356F for 27-30 mins until golden brown. It's all done!