Cut sweet potato (for mash) into small (bite-size). Dice sweet potato (for topping) into about 1 cm (0.4 in) cubes. Soak them in water for about 10 mins.

10 mins later, drain well, and wrap loosely with plastic wrap. Microwave diced sweet potato to heat at 600W for 2 mins. Add honey to it, and mix roughly; set aside.

Microwave the remaining sweet potato to heat at 600W for 6 mins. Mash it with a fork or rolling pin. Add heavy cream in 2 parts. Mix well each time. Strain it. Set aside.

Grease the pan with the butter. Sprinkle bread flour with a tea strainer onto the pan. Tap it with your hand to remove excess flour. Let it sit in a fridge. Preheat an oven to 200℃ / 392°F. Sift dry ingredients (cake flour + baking powder) twice.

Cream the butter until smooth. Add granulated sugar in 3 parts. Mix well each time. Mix it with a hand mixer on low speed for 3 mins until white and fluffy. (If you use a whisk, mix it for 7 mins or more.)

Use a spatula to scrape the butter on the inner side of the bowl. Mix it on low speed for another 1 min.

Put eggs in a another bowl. Beat it lightly until it gets watery. Please warm the egg in hot water until room temperature if it's cold.

Add the beaten egg to the butter in 4-5 parts. Mix thoroughly with a hand mixer on low speed after each addition until creamy. Scrape the batter on the inner side of the bowl, and mix with a hand mixer.

Add vanilla oil, and mix well with a spatula. Add almond flour, and mix it while crushing lumps, until smooth.

Mix lightly the mashed sweet potato to loosen. Add half of the butter mixture into the mashed potato. Mix well until combined.

Put the potato mixture back to the butter mixture. Mix well until combined.

Add all the flour at once. Fold it until powderiness disappears. Use a spatula to scrape the batter on the inner side or bottom, and fold it well.

Put the batter into the pan, and flatten roughly. Top with the diced sweet potato. Push them in gently. Sprinkle the black sesame. Bake it at 170℃/ 338 F for 55 mins.

Drop the pan lightly to prevent the cake from shrinking. Let it cool for 20-30 mins as it is. Remove the cake from the pan gently while it's warm.

Wrap it with plastic wrap twice to seal. Let it sit overnight at room temperature. Done!