Preheat an oven to 200℃ / 392 F. Grease the pan with butter. Sprinkle bread flour onto the pan and tap it with your hand to remove excess flour. Let it sit in a fridge. Mix cake flour and baking powder and sift it twice ; set aside.

Cream the butter. Add granulated sugar to the butter in 3 parts. Mix well each time. After adding all, continue to mix for one and a half mins. Mix it with a whisk until whitish. (for about 5-7 mins)

Beat eggs lightly until it gets watery. Add 1 Tbsp egg at a time to the butter mixture (for 3 times). Mix thoroughly after each addition until creamy. Add the remaining egg in 3 parts. Mix thoroughly after each addition until creamy.

Add vanilla oil and mix well. Sift almond flour in the mixture and mix well. Add sweetened condensed milk and mix well.

Add dry ingredients and fold it until powderiness disappears and gets glossy a little.

Transfer 175 g (6.2 oz) batter into a other bowl. Put cocoa powder in the bowl while sifting with a tea strainer. Fold it 20 times and add 1 Tbsp milk. Fold it until well combined.

Flatten the plain batter. Put blobs of cocoa batter in some places, leaving a little space between each batter. Fold it only 3 times.

Put the batter into the prepared pan. Shake the pan or give light shocks to flatten the batter. Bake it at 170℃ / 338F for 45 mins.

Drop the pan to prevent the cake from shrinking. Leave to cool for 15-20 mins as it is. Remove the cake from the pan and wrap it with plastic wrap twice while it's warm. Let it sit overnight at room temperature. Slice and serve.